i guess we're in part of the game where someone need to challenge mc on his path to Jill, and Tybalt just fit the guy, sure, hes a joke from last 5 episodes, but he can be manipulative like his father, an apple doesnt fall far from the tree. Jill know him longer than mc and for sure know what he's capable of, just like she knows how the rich ones think, so just like the others said, this can be sort of a "test" to prove mcs worth to Jill, now that he drop a confession of falling for her.
as for Jill just giving in on Tybalts demand and not doing anything if what Tybalts blabbering about is true annoyed me, but still, mc and derek is in the wrong here, trespassing and vandalism. a lawsuit for a college freshman?, not so good man.