Oh, come on. She just wants to enjoy her newfound freedom. Also, she desperately needs male approval and attention so she can restore her self-esteem because in her mind, after being dumped, she feels worthless. That's why if Ian says nice things to her about her skills, personality, and especially looks, she wants to jump his bones so much and is really open and aggressive about it. She prefers Ian if he doesn't act like a doofus, because he makes her feel so much better while she's aware that Jeremy only wants to bang her. And she's okay with it, because if Ian doesn't want her (she thinks this way if he acts like a doofus in front of her), so she's going to take any opportunity to restore her self-worth, even if it's just her looks and her body that Jeremy cares about. She's desperate. And it's very sad. I think that Ian can make her feel worthy again, and she'll start looking for more serious relationships again, or not make it a primary focus in her life to pursue countless males for one night. If he doesn't, she may end very badly, step-by-step spiraling down into depravity.
No need to judge her by her weakness. Allow her to feel strong again, so she can show her truly caring personality.