First of all, many Goyim were circumcised as babies so many Geirim only get a Hatafah. And when an adult Ger has an actual bris, it’s almost always done in a hospital where I doubt they allow M’tzitzah B’peh. So the question is probably moot.
Ironically, the whole controversy about M’tzitza B’peh probably wouldn’t apply to an adult. The whole issue that those who are concerned about it has is that direct blood contact with a virus by a baby can be very dangerous to the still developing immune system. There is no such concern at all by an adult with a properly developed immune system (who was probably exposed to Herpes Symplex before anyway).
To answer your question, the man has just undergone an excruciatingly painful procedure. I’m sure there’s no Chashash of anything inappropriate at all.