you're totally right, math has foiled me yet again. but i think this could work! mary beth was in a weird place while at columbia where after being good at things all her life she suddenly wasn't the best or even close to the best at anything, so putting that energy and doubt behind supporting someone else she cared about would have been not necessarily the most healthy choice but definitely the kind of thing she would do! his relationship with the actress playing allie wouldn't have been an immediate dealbreaker for her, but the intense outside insistence from the rest of the world that the two of them were meant to be together and he should ditch that stupid nobody girlfriend of his for her would have bothered her a lot, especially since, like you mentioned, she would have just been trying to finish at columbia and live normally. she wouldn't have deliberately tried to make the break up dramatic and messy, but she can be pretty blunt and lash out when she's feeling cornered or frustrated. were you thinking they'd be civil by now, or not so much?