That guy owes you a new phone, period. If he is going to do something so stupid, he needs to learn the lesson that there are consequences.
When my wife dunked her iPhone in the toilet, the Genius Bar employee told her that the phone might be fine, or some things might be broken, or that it might seem fine for a while, then just suddenly stop working. Its like a time bomb, you never know if your phone is going to just stop working now. So the guy should replace the phone.
The M8 is water resistant to some degree (but I don't know about total immersion). I'd dry it for a lot more than overnight. If any water go inside the phone, I don't think drying overnight is sufficient. And any remaining water is a possible point of short circuit or corrosion. Power off and dry it for a day or 2 more.
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Phone: AT&T HTC One (M8)
Tablet: Samsung Galaxy Tab Pro 8.4 (WiFi only)