One very good way of explaining this to a player is by asking him to justify his actions with real world examples. Obviously, you're dealing with the physics of magic so this rule can really only carry so far, but if you are dealing with a fairly intelligent, fairly rational person, you can describe this situation as such:
You are in a gunfight, and someone shoots you. It penetrates your body armor and hits you in the chest, doing damage to you. You apply a piece of even stronger bulletproof duct tape to cover up the hole after you've already been shot. how would you ever in a million years also assume that you would magically be healed of the damage as well?
If your player can give you good enough reasoning for why this feat or spell would work that way, I'd say think about it, and then make a judges ruling. But in this circumstance, let your player know he is wrong. very very wrong.
You are in a gunfight, and someone shoots you. It penetrates your body armor and hits you in the chest, doing damage to you. You apply a piece of even stronger bulletproof duct tape to cover up the hole after you've already been shot. how would you ever in a million years also assume that you would magically be healed of the damage as well?
If your player can give you good enough reasoning for why this feat or spell would work that way, I'd say think about it, and then make a judges ruling. But in this circumstance, let your player know he is wrong. very very wrong.