She is looking for someone to entertain her without having to announce to the world that she is a bad person for dumping the BF. She will get bored with you quickly and find another gym guy if you get too friendly with her. BF is not changing. She probably wishes he’d do something to make her angry so she’ll have a girl reason to dump him. Looks like you have raised up her status in your world too quickly. She has to earn it. Even though you told her you can’t be friends with her you have been. Go no contact. If she persists, while you are finding more plates, stall her for a few weeks with aloof responses to her messages. You’re going to have to distance yourself from her. She is probably into you and dying for sex. She will sleep with someone with or without the BF. If you want it to be you, put it into your head that there is no exclusivity with her and the relationship is only sexual. She will respond to being one of your plates better than being exclusive with you. I am assuming she lives with the BF. She has a lot to lose in her sex quest. Make it easy for her.