Omg almighty, why are you trying so hard to make this a thing mate, this is so far fetched. Never in a million years can this even be a thing. Your theory is so far-fetched mate and if I think about it in a scenario and how DPC develops I just think these theories that you post are just a dead end. Like my man
klep12 said keep dreaming
I'm not trying to "make it a thing", I'm simply speaking to what I've observed in this game. It's a theory, sure, but I don't see it as being that implausible.
For starters, who else could be the "friend" if not Jill? Jade? Cathy? Jill's sister? I would consider each of those to be more far-fetched than Jill because there has been zero indication that Bella is in any way friends with Jade and Cathy, and if it were Jill's sister, why would she still feel guilty after all these years? Is it the guilt over doing something with Jill's sister years ago that led her to be a protective sister-figure for Jill to make up for it? Possibly, but I just don't see a line getting thrown in like if it didn't have relevance to the present-day story and a character within it. It is, to me, like how Maya and Josy's lines about their relationships were clearly going to relate to a character we already knew and that their respective relationships would eventually be revealed, which they were.
Also, I don't see it as being that out of character for Jill either. Sure, her personality is that of a sweet, kind, innocent person with a positive outlook on life and people, but that doesn't mean she couldn't intimately kiss another woman in the right circumstances, like getting upset over a guy and taking comfort in the arms of a close friend. Friends sharing an intimate moment is something that happens and doesn't have to lead to anything more. It can easily be a one-and-done thing that is quickly forgotten about and moved past, and I know of what I speak because it's happened to me before. So I could easily see Jill think nothing of it and brush it off like it wasn't a big deal.
From what I've gathered about this game, DPC is very good at putting in these little breadcrumbs that set-up future plot points and story details, especially when it comes to the main girls and characters that are key parts of the overall story arc. This is why I don't think Bella's like was a throwaway that means nothing, but it also doesn't mean that I think it indicates a past or future relationship for Jill and Bella. I'm not saying that the MC will find himself in another threesome and I'm not trying to "make it a thing", I am simply saying that I believe that line referred to Jill. Nothing more.