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