I remember that there were discussion prior to Ep 5 about who the "friend" was, and people often pointed to Stephen and Jade and that this was referring to her turning them down, but even at that time I doubted she was referring to them because she certainly does not seem to be freinds with them. I suggested at that time that it might be Jill since she's the only friend we only know of and I got a lot of flack for it, but Ep 5 has added a lot of fuel to that fire.
Again, I wouldn't say they went as far as sex, but something sexual may have happened and it would be the biggest misdirection if it didn't because I honestly think it's difficult to come up with anything else that to explain what they were referring to that makes sense.
I don't think so, because she's addressing Bella's physical action of turning away from her which feels like an action to stop herself from doing something with Jill. Therefore, it feels like Jill is trying to say that this thing Bella feels she might do is "not going to happen again".
This is why I would say to those who think she's referring to a similar incident like the one with the MC that it isn't likely to be about that. If Jill knew she did this sort of thing, she wouldn't invite the MC to bed with her if there was a likelihood she'd do it with him. So whatever happened between Bella and Jill must have happened when they were both awake and conscious of it.
I'd still say that Bella and Jill making out and possibly getting near to going all the way wouldn't be out of character at all. We know how "traditional" she says she is, but this likely means that she's quite sex-starved and perhaps only acts this way because she thinks she has to, but she's repressing a wilder side or at least one that wants sex but she fights her urges. We also Bella and her like to spend evenings together drinking wine and they've both suffered some emotional setbacks in their relationships, so a night of drinking when they're both in an emotionally vulnerable state and are both feeling sex-starved is like the perfect catalyst for something to happen between them.