I'm sorry but I disagree, I consider Jill indefensible not because she did something wrong or unforgivable, but because she suddenly behaved like a total naive being completely maneuvered by Tybalt, who is certainly not Moriarty, an incredible strategist
from a girl with a very strong moral compass, so convinced of what is right or wrong that she decides to go against what Mc had expressly asked her, she becomes a sucker who believes every word of Tybalt, accepts his ridiculous proposal, and does not verify anything than what you have been told. she never asks MC for his version, never.
the icing on the cake is the picnic in which the grand finale was to declare to Mc that she could no longer see him, without having asked him anything yet...
a series of absurdities lined up for the sole purpose of giving comic segments
as far as Jill knows, what would stop Tybalt now, other than a broken heart, from reporting MC? after being even publicly humiliated. and if Jill knows there is obviously no such threat what is she going to do on the third date?