The good news is that he's
not going to a Rhino shop
Take your truck to a car wash and use a pressure washer to clean it.
DO NOT prep the truck yourself, you will be making a big-big mistake. Typically, the drain function is not affected because the product tends to "stack" instead of flow. In almost 6 years, I've NEVER had a customer return with a drain problem. However, if for some reason your drainage is affected, you or the dealer can use a Dremel or a small drill bit to ream out drain areas.