Some people don't deserve good behaviour

To be fair it did seem to look like he was willi g to flirt with her

While the " I have a boyfriend " people tend to be annoying, this one isn't. Some guy messaging you from the club is USUALLY trying to flirt with you. And if all he wanted to do was give back her purse he could have been like " hey are you Alice? I have your purse " and started with his intent rather than trying to see if she remembers him.
And if he refuses to give back her stuff to be petty she can call the cops on him for theft.

That's a dick move. Saying I have a boyfriend makes sense in this context because they were talking about a club.

"You dropped your shit and left it"
"I have a boyfriend!"
"That's nice, you want him to come get your shit?"

True.. some random guy from club might sure flirt with you.. but I have a problem when she says "He must get it to her".. not the other way around..