Thats one thing we were thinking as well, kinda surprised water was able to get in there all and all. Ill pass the info along and see what happens.
4-foot of water is well above the CVT intake @ 32"..in fact 48"s is 6" above my doors of my Zs so..the entire body...and engine air intake was under water if not snorkeled...if that's true, the belt being wet isn't surprising at all. What is surprising is that it continued to run with the engine air intake under water.
There is a little drain on the cover. Then like Cosmodan says, run it in neutral at enough of an RPM for the wet clutch to engage the primary and move wind and heat through it for a while....and tell him to snorkel it before going in anything over 2-foot.
The down side for him is..now that the belt has slipped, it's now glazed.. and will start slipping under load even not wet. So tell him to expect it to fail and better get a new one ready to go.