"breath play" is definitely edge play and really not to be done by people who don't know what their doing. So i would really do some communicating about your concerns, because you're right, it can be very dangerous. Keep in mind, even though it's scary to see and think about - this was done with her full knowledge, after negotiating with him, and consent. So as long as he's knowledgeable about the safety issues and knows how to be safe, i would say doesn't appear to be "harming" her. Remember Bdsm is about Safe Sane and Consentual (even edge play can be done safely). So even if he appears to be "hurting" there is a different between that and "harming" her.
i've been in the Bdsm lifestyle for 10 years and am very active in my local community. Please feel free to ask me questions. Also, read, do research, have a sit down with both of them and talk to them about this aspect of your relationship. As you have not consented to any of these activities, it appears he has taken pains to not "harm" you.