The first thing you need to do is to stop worrying about whether the guy she is seeing now, as well as the whole set of her future partners throughout her lifetime, would get hurt by her. After that, stop attaching any hopes as to the course and outcome of her currrent and future relationships with anybody other than you.

After those two steps have been taken, we can discuss your next steps.

Just to mention a curious disconnect in your account: you report both very low. Frequency of sexual activity and high level of sexual chemistry at the same time.