I believe the best thing would be is to pull back and watch with eyes open. Sounds to me like she's playing him, if she's picking your relationship apart...not fair....but if you say something, well, simply said, you might want to know now if he's going to buy into this, cuz if he does...then your better off knowing now? Right? You wouldn't want to invest years of emotions, trust and faith in someone who cannot give that to you. If he's tempted that easily, then he'll do it later, or....he's done it before? The balls really in his court, and theres nothing you could do, or should do...you want someone who is mature enough to return your love?
Just my thoughts