So basically, lets say there is a nice boy named Jake.
His father left him when he was young, and he's a bit scatterbrained, but he's relatively happy and full of creativity.
But for years he has had to live with the guilty, shameful and fearful emotions that came with watching his mother physically/mentally abused by this horrible boyfriend that she met in the 2000s.
Jake needs help.
His mental health is suffering due to this evil "boyfriend" who really didn't need to exist at all.
Jake's mother often works 14 hours just to make ends meet as the boyfriend lazes around the house or barely works at all.
Jake has been robbed of so much precious time with his mother and is in danger of either falling into a deep depression or snapping.
Over time, Jake has has had horrible thoughts run through his mind and everyday becomes more tempted to act on them.
Jake wants to ask for help as to how he could get his mother's abusive boyfriend to leave their home and leave them alone forever.

If it's physical abuse, try the authorities. Jake could also see if his mother is willing to share his view on the boyfriend, and work on a solution together. If Jake has any other family that could help, try reaching out to those? I can't imagine any family who would willingly let this happen.

Jake's family is pretty fucked tbh.
But I'll let him know what he needs to do.
Thanks.

I hope he finds a solution. Jake sounds like he could use a happy day or two.

Always talk to the police about voilence

Violence

hidden camera to record the abuse then call the cops.

Jake needs some proof. Jake's mom, if confronted by police, will most likely deny it and because the cops have no proof, they would leave and Jake's mom would most likely get abused even more. If Jake can get video/picture evidence of the physical abuse and recordings of the audio and brings it to police, he might be able to get charges laid against him. If Jake's mother doesn't press charges, there isn't much he can do. Make sure that Jake doesn't attempt anything rash against this asshole - it will backfire. Jake's mother most likely feels that she can't leave and Jake needs to convince her, if that is the case, to leave. Get his family informed and involved. Don't let Jake attempt this by himself.