I dont know... Jill doesn't give me the impression that she bats for both teams.Its pretty obvious they did stuff. Which made bellas library scene where she said "I shouldn't be thinking about a student like this" "or doing something with a close friend" "I'm sorry" make since (I missed it cuz I simply chalked it up to she was talking just about mc)
Im not saying its an impossibility, it sort of fits I guess, it just feels like this is something a bit deeper than Jill having possibly made a move on Isabella or vice-versa.