The character development seems fine to me. The revenge, not so much (in fact, I'd only be interested if all sexual components were left out). Anyway, if I were going to be interested in the MC having a relationship with Tiffany, she should break up with her boyfriend first before the MC and she start the relationship (the order is, break up first, then start the relationship).
Which might happen, from what I'm picking up from the mc when reading. Is hes a kind soul so I doubt revenge are in the cards im just spitballing. Secondly she seems the loyal type so I think she would break up before ever cheating on the man even if she felt hurt by him. We will just have to see you might of hit the nail on head with that though it seems she will leave him first and then we see her develop more as a character rather than the shallow bimbo we think she is