But that theory is still predicated on Jill knowing or at least suspecting there's a thing between Bella and the MC, and that's certainly not the case. Also, in that theory of yours, Jill would almost be saying that her tempting a guy away from Bella is what's "not going to happen again", and why would she think that if she has no clue about Bella and the MC?
So I think the idea that Bella's husband made a move on Jill doesn't really seem to make a lot of sense.
For me, it's all about those renders where they appear to be looking intently into each others eyes.
Bella breaks the eye contact in a way that makes it feel like she thought something would happen if they kept looking at each other like that. After all, they've had a fairly nice, relaxed evening with a few glasses or perhaps even bottles of wine in an intimate, canled-lit ambience with a guy they both like, and who may also have played the guitar for them.
That seems like something that might make them both feel a little in the mood and it may be similar to how something happened before, hence why Bella ends the eye contact and why Jill tries to assure her that, "It's not gonna happen again."