Burke (or Jade, don't recall which) mentioned trying to spice up their relationship and mentioned Bella. Bella seemingly implies that she did it.
I actually found this post that basically describes what went down, it was a few hours ago lol. This threads moves too fast to keep up with
The thing is though, we don't actually know when the Bella/Jade/Burke stuff happened, as mentioned above. The husband could've left when he found out and Bella hasn't done anything since because of regret. Thats why I said Bella has cheated more than once, potentially.
I had to do a quick skip through of the game on her route to confirm the timeline here. I don't think she did anything whatsoever with Jade/Stephen. All quotes of guilt equally apply to the MC and Stephen explicitly states he never slept with Bella. Timeline is this. Burke invites her in Ep1 and says they can meet Friday night. She needs to bring wine.
Then, you get the option to either go to the DIK party or stay home with Maya. During the DIK party it's explicitly stated that it is Friday night (the conversation is about how packed it is and Rusty confirms the day is Friday) and you then go home for your Josy date at the weekend. So at this point Bella has been on her Burke/Jade evening.
Later on you meet Bella and get invited to dinner, get drunk and ruin it. You kiss her she says she can't do this "Not with a young student, not with anyone!" (that's a direct quote). She then goes back to bed and masturbates and thinks "How long has it been since a man did this to me?" "Three years?" Now that doesn't prove she didn't do anything with Burke/Jade just that it didn't... inspire her to masturbate like MC did.
Ep3 rolls around and Jade and Stephen argue. She says they would only spice things up "with each other" and Stephen retorts that they "Well we tried that already, most recently with Isabella! She was clearly already in love." He brings up Isabella again and Jade gets made and he explicitly says "I haven't even fucked her and you're acting like I did". From here you gain Bella's trust at the cafeteria and she then tries to actively set you and Jill up. She pushes Jill to date you, says she likes you and texts Jill when you're in the library so she can come see you.
Stuff happens eventually you go see Bella in the library after asking Jill on a date and her inner monologue the stuff about "Married women don't fool around with young student, they aren't intimate with friends either... but that was different, It doesn't matter I've stopped it. Please James forgive me." You kiss her and she asks "why are you trying this again."
See to me, this implies the monologue was referencing the MC as the "friend" and she's "stopped" anything happening with him. The flow was you don't fool around with young students, they aren't intimate with friends *either* meaning the MC is both a young student and a friend. She's saying there's double the reason to not be kissing the MC.
Nothing anyone has said indicates she did anything with Jade/Stephen. Stephen explicitly denies it and Jade doesn't mention Bella when rattling off people Stephen cheated with (namely Cathy). They tried to rope Bella into something but she wasn't interested. Bella was into the MC and felt guilty over that, not being with Stephen.