That's because people like that view their partners as a possession and not a companion, in this situation he want to date her to proclaim to others he "owns" her like you'd flex some luxury shoes or watch or shit like that.Surely blackmailing the person and forcing them against their will will surely get them to like you more. I just can't with this kind of mentality
Other option is it being an opportune time to come clean. But there's still the whole step-bro thing, although their parents might not care but I imagine other students and faculty staff might.She has to do is say no and tell them what happened yesterday. Everyone will immediately see that he's just lashing out because he got turned down and he's an incel that thinks that for some reason he deserves her.