In fact Bella is a slut.
Right from the start, we cheated on Jill with her (sort of). We go to see Bella to thank her for talking to Jill about us, which made it possible for us to invite Jill on a date, and it almost ends in a fuck on the library floor.
So early on, Bella does stuff with the MC even though she knows he and Jill see each other.
Either Bella dreams of a threesome with Jill and the MC (there was already something between her and Jill). Either she's a slut with her best friend. Or either she just uses the MC as a sex toy to get rid of her husband's memory and move on. These are three options that are not mutually exclusive ...
Last possibility, on the contrary, she is the best possible friend for Jill. She knows that Jill is the queen of blue balling, and so she sexually occupies the MC while Jill decides to give us her cherry.