Given just how down Jill was on her own sexuality, I don't think anything happened between her and Bella and/or James. But it's hard to guess what DID happen in the face of so little evidence. What do we actually know about Bella and her marriage?
- We know Jill knows something about what happened, but she considers it private information that would be a breach of Bella's trust to share.
- Bella has a locked room in her house (and Jill knows nothing about it)
- We know from Bella's inner monologues that both she and James did something that could merit forgiveness.
- It's strongly implied that Bella was inappropriately intimate with a friend at one point, but put a stop to it
- We know that Bella hates adultery and almost freaks out when she first spends the night with the MC, but seems calmed when she realizes she feels more than just lust towards him; this implies that a) she sees adultery as purely driven by lust, b) she was not an adulterer before fooling around with the MC
- Bella was wild in her youth, but that apparently changed sometime after her wedding
- Young Bella only found one guy to fall for (but might pursued multiples if she'd found them)
- Modern day Bella clings to her marriage (until the MC wears her down) despite James being absent
- Bella wants the MC to commit to her before she's ready to become his lover for real
I don't see any plausible scenarios that can easily connect all those dots. My best guess at the moment is that Bella fooled around a lot in her youth, but she consciously put a lid on her libido. This strained and ultimately broke the marriage since James liked her uninhibited side. Bella regretted her inaction, but with James gone she had no choice but to bottle her up libido even further... until the MC came along.
That ties in with a lot of the points, but it doesn't explained the locked room and it's pure speculation that Bella's attitude towards sex would change so drastically after marriage. In the end, I'm just guessing wildly, and there are other theories that are just as good.