We know Jill & Bella are best friends, but not Bella's only friend shown in game: she's also friends with Cathy (hence going to Bella's house after that cluck) & formerly Jade/Stephen (they wouldn't have propositioned Bella if they weren't close with her). Bella turned them down, which presumably ended the friendship.
Propostioning Bella doesn't mean they're as close as friends. They're work colleagues and probably close enough in that sense for Stephen to think it was a good idea to proposition her since he could potentially rely on the belief in "professional courtesy" that she wouldn't mention it to other people should she approve or reject. We also don't have any indication that she Jade are or were friends at any point.
The young student is clearly the MC, who she made out with in Ep 2. Bella wants to distance herself from him for Jill's sake since she's trying to set them up (but continues to fail abysmally on that front thankfully).
The intimate with friends part could easily refer to the Stephen/Jade proposition, which Bella stopped, causing Bella & Jade to have a falling out.
Bella clearly hopes that Jill and the MC getting closer will keep her feelings for him at bay, but again, we have no clear indication that Bella is or ever was friends with Jade and Stephen.
No? Bella is upset about 'something' Jill is in the loop on, Jill tries to reassure her & gets rebuked for bringing up whatever they have an understanding about. End of actual info.
All the staring can simply be put down to Bella being upset & Jill being pensive about how to deal with it. Nothing in this scene says 'it' has anything to do with an intimate encounter between Jill & Bella. Some folks are just projecting what they wish for me thinks.
But what is it specifically about staring at Jill that made her do that? Guilt over what she's been doing with the MC? Possibly, but if a player did not go see Bella in Ep 3 or stay with her in Ep 4, then she's done nothing with him besides one kiss, so that rules that out. It seems like something about this particular situation she finds herself in that not only gets her quite agitated but also makes Jill realise it relates to something that happened before. Jill even said she would have slept on the couch which would seem to indicate that she felt it best to not share the bed with Bella.
So here we have a situation where the 2 of them are in bed together, Bella is clearly uncomfortable about it, Jill realises this is a reminder of a past incident, and Jill acknowledges that she would have slept on the couch to make things easier for Bella, so it seems likely to be something about this specific set of circumstances.
Given you penchant for theorizing I'm surprised at your lack of imagination in this context?
The most obvious something else would be whatever caused James to disappear, which Jill clearly knows about. This would also explain why Bella tosses James picture during that Bella/Jill bedroom scene.
Because there isn't really much else to go on here and anything else would either be wildly crazy (which certainly can be in DPC's wheelhouse at times), or doesn't line up with what we know so far. Maybe Bella has night terrors and physically lashes out during her sleep causing grievous bodily harm to whoever sleeps next to her and James got tired of it so he left. This would explain why Jill was going to sleep on the couch and that this something wasn't going to "happen again", but then she wasn't bothered about sleeping with the MC in his room, so if this is something that happens with her, you would think she'd be less willing to stay with him.
That's literally the only other thing I can think of but even to me it sounds ridiculous and I don't think DPC would ever make it something like that.
Or they're talking about the Jade/Stephen proposition, the triangular entanglement blowing up her friendships.
I don't believe Bella would tell Jill about their propostion to her.
Or a hundred other things - DPC is hardly going to spoil the suspense by giving the players all the facts now is he?
It does seem somewhat obvious, yes, and DPC generally doesn't do obvious, so perhaps this is a wild goose chase, but as of right now it does feel like it points in this direction. If it indeed turns out to be something else, then it would have to be something that connects all these dots in as clear a manner as this current theory, but right now I simply can't fathom what that could be.