This guy is too sus. My guess is that he really needed someone to pretend to date, but he chose her because he probably knew that whoever he brought would be mistreated. So basically he's killing two birds with one stone here: he's using her as a shield while also enjoying watching his friend's ex girlfriend being humiliated. It's pretty clear he set a trap for her using that glass thing and there's no other reason she had to be the one he brought to see his stepmom, other than to use the opportunity to do a little stupid vendetta for his friend's sake. Still I hope I'm wrong, since this guy's probably the endgame.