It may not be arson, but MC was clearly trespassing and doing some illegal shit. There will be multiple witnesses. The bottom line is that even if the MC has a decent shot at court (which I doubt, as does Jill), Tybalt could drag him through a lengthy litigation process and just screw him on legal fees. That's basically how it is. The rich has such an immense advantage over some broke kid when it comes to legal matters.
Jill cares about MC, but enough to fight a court case on his behalf? They aren't a couple yet, and this type of thing would be handled by her family. Even if she can do it herself, her family would find out and not approve of it.
The thing about DPC as a writer is that he always tries to ground the characters in reality a bit. Not necessarily the plot arcs and what happens throughout the story, but the characters' reactions to things that are happening around them.