Ok, I thought about it lately and I agree with you: that scene in the library with her waiting for MC take place a little before talking with him, after Bella says "I tollerate you" and "It's Bella", you can say that she consider him as a friend (with benefits, but just because she's sexually attracted to him, there's more than that). She "stopped it" because she knows Jill is interested in him too and she decided to step aside, that's why Bella text Jill letting her know he was in the library, even though you can see she's still jealous when she see them together, she did it because of Jill.
Besides, and that's why I believe her character won't be ruined as we fear, the most revealing scene about what happened until now is the scene at the pool.
Do you guys really believe that Bella could think "I'm doing it... I'm finally doing it" after she fooled around with another person before? That was the first time she did something sexual with someone who's not her husband, therefore we have 2 possible answers:
1) That friend is the MC.
2) DPC made a mess.