If by "Why would Jill allow Bella to do that?" you're referring to Bella calling the MC, well obviously she wouldn't know that's who she's calling. We also know that Jill is a people pleaser and doesn't enjoy confrontation, so if Bella wants the MC to be there then she's not going argue against it because it's Bella's choice. Also, she didn't exactly have a problem hanging out with him in the library and Tybalt even saw them together, so why would she have a problem here? Perhaps she thinks that the conditions only apply to seeing him alone and not in social interactions where there are other people around, in which case a doubles tennis match qualifies. Also, she might see this as an opportunity to mend the rift between the MC and Tybalt and thus make Tybalt rescind his ultimatum/blackmail.
Who do you think Jill would expect Bella to call? As you've said in the past, Bella doesn't seem to have a lively social circle at the moment.
Also, Jill DID have a problem in the library. She had to be talked into staying by Josy, then roped into D&G by Magnar. On top of that, she probably would have left with Tybalt if not for Sally. Jill knows full well she's "supposed" to spend time with Tybalt and not with the MC.
And before you say "Bella will be just like Sally," keep in mind that in that case Tybalt decided to leave rather than continue to press Jill to toe the line. If he leaves, no doubles. If he stays, Jill will have to either call Tybalt's bluff on her own or knuckle under. Those are the two scenarios I just laid out.
Yes, Tybalt has been the butt of many a joke, but that's the point. He is that archetypal upperclass twit who thinks he's better than the blue-collar protag, but the protag keeps getting one over on him in various comical and humiliating ways, and that's Tybalt's role here. He's not likely meant to be any sort of serious contender or antagonist, he's meant to be the joke and it can it happen as many times as it wants. In Wile E Coyote cartoons, you always know he's going to get humiliated at every turn, but it never stops being funny, and that's Tybalt.
It stopped being a joke in Episode 6 when Jill bought into his ultimatum! If Tybalt remains the one-note buffoon, then Jill really is the sheltered fool people often accuse her of being. That's worse than Tybalt being a viable threat, at least in my book.
If this is the moment when Jill calls his bluff and humiliates him yet again, fine by me; I never liked that plotline to begin with. But in that I case I expect a real relationship with Jill (or fireworks if he's been sleeping around), because there's nothing left to stand in the way. I'm sure as hell not going to play along when the MC tries to climb onto Jill's balcony at night because "we can't be seen together."
That means no more side chicks, no more rain checks to bump Jill for other plot points, and no more introspective looks at her polaroid. Do you think we're ready for that? I don't.
Oh, and on an unrelated note, am I the only one who was rooting for Wile E. Coyote because I got tired of watching the Road Runner humiliate him?