It's definitely complicated.
Bella knows that Jill is dating the mc, she even asks Jill how the dates go (we saw that when they were doing Yoga in Bella's house), but obviously Bella keeps her relationship with the MC a secret from Jill.
Bella herself doesn't even know what she's doing with the MC, and each time they do something she chastises herself and feels guilty once the urges subside.
She definitely isn't doing it to hurt Jill, and if she rejects the mc over the Cathy business and Bella says that he needs to be good to Jill, the mc calls her out for her hypocrisy. She says that's just all the more reason for them to be parting ways.
If the Cathy situation doesn't occur and Bella doesn't end the relationship with the mc, that doesn't mean she doesn't have mixed feeling about messing around with the mc while knowing he's forming a relationship with Jill, it just means she's weighing it up differently. Cathy just tipped the scales.
Sure, but you're breaking the illusion.