I'd like to know what you mean by "confront him". Would this be in person or not? As in person there's the chance of it escalating into a fight, even if he is the only one throwing punches, that won't help. If neither of you are at the high school I would say ignoring it is best. I know you said some of the people from that school are still in her life, but if they are true friends they won't believe, condone, or be swayed by it in any way. As for what you can do to make this better for your girlfriend, other than reassurance that you don't believe it and aren't going anywhere, you could take her out to distract her from it. Doesn't have to be expensive, picnics, walks, beach, whatever you guys like to do.

Depends on the kind of stuff he's saying. Sometimes you need to kick someone's ass to get them to mind their own business.

Dude they're highschool age, thats a terrible idea

Or you can sue him for defamation.

Or slander. https://en.m.wikipedia.org/wiki/Defamation

Okay so theres been 3 suggestions:
Dont worry about it its just words
Beat him up
Pursue legal action
These are high schoolers you psychopaths

Highschoolers can be pieces of shit human beings, they are emotionally unstable with underdeveloped frontal lobes and the inability to recognise sensible action or act with emotional maturity.
Some are more mature, but getting through to point out that "this slander is unnecessary bullpoopie and it's hurtful to others' reputation which can cause severe emotional and possibly physical distress", is gonna be more of a mission than punching them in the guts or threatening them with the law.
The things teenagers mostly care about is (1)themselves, and indirectly (2)not to get in deep trouble with their parent's - who will possibly prevent them from seeing their friends as punishment, thereby running at risk of decreasing their social status and self-worth, thus looping back to (1)themselves.

"Some of those people are still in her life."
In what capacity? If they're friends, they should know her better than that. If they're not, then it's irrelevant because she won't even remember what any of them look like in about 5 years.
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This is a risk that comes with pretty much any relationship. The subtext here tells me he still has some emotional affinity to her-- whether it's just childish bitterness, or petty jealousy. It takes a decent amount of effort to work your entire life around ruining someone else's reputation. Clearly he is still VERY hung up on her.
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At the end of the day what he says does not matter and carries no more weight than she allows it to.
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YOU swooping in to confront him will likely not end well. At best, it will escalate into a fight that could ruin either of your school-careers or potentially criminal charges. What is it you're planning to do and what do you hope to accomplish?
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Is there a reason she does not confront him herself if she's this upset?

She shouldn't be worried about shit that comes out that dudes mouth. If anyone cares about what another person has to say about them that isn't getting a reference from that person clearly cares about that person. In other words, if she cares what he says, she still got a soft spot for him. Maturity is knowing what you want and not worrying about anyone except yourself and your loved ones. Period. Anyone who wants to argue that is clearly not mature.

The problem as other have said is: you’re in high school. You can do the “adult” thing- but that hinges on your girl being similarly mature. If she has a high school mentality- she likely won’t respond well to being mature. If you confront him you must be careful. I don’t know who would kick who’s ass- but I know that it seems like fighting isn’t handled like in my day where ass whoopin’s were on the menu. I don’t want to advise you to do something that could get you hurt or in trouble. Verbal confrontation hinges on his maturity- and it sounds like he’s probably a child. So he’ll likely goad you and think the whole thing is quite funny, rebel against perceived authority in a teenage way by taunting “what are you going to do if I don’t?” And... well, at that point you’re feeding trolls more or less because your genuine emotion or appeals to humanity will likely just get his little troll dick harder and egg him on to fuck with you harder- either just because he’s a troll, or as a...

... power thing. To show you that he doesn’t have to listen to you, that he has the power to make you miserable or upset. He waited to break up and her to change schools to spread rumors. Probably things like “she let me do this...” “she’s so dirty we once...” so on. He’s a little pussy. He waited so she couldn’t defend herself and he didn’t have to worry about facing her regularly. He wants to feel like and be thought of as some “big man” and spread his bullshit legend and figures it doesn’t matter since she’s gone so he may as well. He’s a sad pathetic human being who is small inside.

So “high school rules” you could confront him- maybe fight him. You could get involved in some elaborate immature scheme to spread lies about his small dick, or how he used to always get horny after seeing Carson Daily anywhere- or how he owns his own dildo he hides in his room and uses to butt play with himself, etc. you could get a plan going to attack his obvious emotional weaknesses and insecurities. But- it’s 2019. You could get in big trouble over that, especially if he flips out- not to mention it’s childish and should be beneath you.

So what then? Well.... maybe look into anti bullying or other laws and rules. I have to imagine in today’s climate- spreading sexually vulgar things about a girl as a teen is probably grounds for some type of discipline. I’d recommend using the system on this one. Take it to a teacher or principal with as much proof as you can get of what he’s saying and that it is him saying it.

Unless you know him well enough and are a very good speaker and improviser, and can keep cool- confronting him might make it worse. It’s high school. Unless it’s changed a lot, asking a douche bag to stop hurting you and your girlfriends feelings didn’t usually go well in my day unless it was backed up with some force- and I don’t advise you use force. So turn him in and let the teachers deal with it. Also talk to her and to her people. Do damage control as much as possible. But if you want to stop him, and make sure that you’re covered- find a way to report his behavior to adults. The truth about being an adult is- at a certain point even we tell the “adults.” Why feud with a neighbor when you can tell the landlord? Why yell at the guy parked in your driveway when you can have his ass towed? Why chase the guy who hit your car? You call the cops. Unless you like drama for the sake of drama- have someone with the authority to handle it do so, and go back to living your life instead.

I know I'm mature enough to know that fighting will just make it worse and the persuing legal action thing would mean including the parents cause we are 17,hence not an option so I think I'll just talk it out

Good luck.

Well I did it

I was calm af and talked normally to the guy and he did the same but he wanted to tell me his side of the story which was bullshit but when he got to the part where he breaks up he got weirdly angry and started abusing,I waited for him at realize that and for him to act a bit natural but well he continued,I roasted him a lot,well I had to...and of course he asked me to fight,but he was jealous of me a lot so he hanged up quite sobbing

Act a bit mature*

I hope that it turns out productively. I suppose you will have to give it time and monitor the situation to tell.