Ask Dan & Jennifer

OK, I don't know why, but my friends all seem to come to me when they have questions about sex. I have a friend who is new to BDSM and has been really diving in deep from the start. Now he's telling me this Domme he's been with has been doing breath play with him and now she wants to put him in a tub and hold his head under water until he does whatever it is she wants. He told me he's scared, and I would be too! I'm more into some milder play, so I'm really not familar with the erotic asphyxiation stuff. I can see just doing it on dry land, but doing it in the water seems, well, dangerous. Any comments?
I'm just thinking if you accidentally inhale while on a dry bed or something what's the worst that happens, you get punished for breathing. But under water you drown..
I just told him I wouldn't do it if I wasn't totally comfortable and trusting this woman. Afterall, he's only been doing this since early last month, and how well can you really know a person in like 6 weeks? I told him to make sure he's expressed his limits to her and that she's understanding of them and willing to respect that. Also, I wanted to know.. if he does decide he wants to do this, what does one do in lieu of a safe word? Make a hand sign?
Thanks for any comments on this. I dunno.. maybe I'm just living in the dark and this is common... let me know.