I haven't tried this personally (fortuneatly I haven't had the need) but I have been advised that a good soak (like 15 -20 minutes) in lukewarm water with a few drops of dish soap can help kill mites. The dish soap apparently breaks the surface tension, preventing air pockets from forming around the mites while submerged so that they drown. I doubt that alone will cure it, but it may help when combined with other treatments. I suggest he disposes of everything that was in that enclosure but the snake, and make a temporary switch to white paper towels as a substrate until he's rid of them (that way he can see the little buggers and get a better sense of how bad it is).