Here was the situation. Opponent had Protectrix ("Reap: Fully heal one friendly creature. If you do, that creature cannot receive damage this turn.") Opponent also had another creature ready. That creature had no damage markers on it. Opponent reaped with Protectrix--then claimed he was healing that other creature, and since after healing it it had no damage, this counted as "fully healing" that creature. He then used that other creature to attack one of my creatures, and did not allow for his creature to take any damage.
Opponent's assumption was that you can heal any creature, damaged or not.
My assumption instead would be that you can only heal a damaged creature, and that Protectrix could not be used to protect that creature from damage in that situation.
What say you!?