Is this safe?

[K. in Cleveland]

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.

[The Beautiful Kind]

I hate to be a party pooper, but your friend shouldn't do it, it's not worth the risk. I mean even if he inhales a little water he could contract pneumonia and get really sick.
So, no, he shouldn't learn how to say "I'm drowning!" in sign language. That domme needs to stick with playing with rubber duckies in the tub.

He should fill out my BDSM fetish checklist to share with his play partners. It's available on this site here:
The Beautiful Kind’s Ultimate BDSM, Fetish, & Kink Checklist

[K. in Cleveland]

I'm glad to see I'm not "behind the times" or just plain ignorant of kinky stuff... My first reaction was not horror but kind of like that feeling right before you jump out of a plane...hoping the parachute is open and that you'll live to jump again... but I could just see jumping into this and not having a chute to open. Thanks for confirming my initial thoughts (go with your gut!)