At 16, there is not a sexual assault charge (
ageofconsent.com). However, being her parent, you can enforce who she sees and when she sees them. If after being informed that he is to have no contact with her, and he violates that, you can file for a restraining order (or something similar). If she is doing illegal things with him, he can be charged with contributing to the delinquency of a minor (or something similar).
Thank goodness there are parents who care!!! Good for you!