First, tell her no. My wife and I always made certain we were together at the second meeting, if not at the first, with any such potential situation. If he/you do not wish to end his and her relationship completely (so you can help her together), then invite her to meet with the both of you, and get to know you as a couple. Of course, if you/he decide to never see her again, then tell her so, and do whatever you have to do to make it happen.