An interesting deck build was played by one of my players the other day where he was healing me (his opponent) and due to an ongoing card effect (forget the specific card) he was causing me twice the amount of damage he was healing.
In essence if I had 5 damage on me he would heal me for 5 but cause twice the amount of damage he healed to my hero due to the ongoing card effect. I can't see where it would be against the rules to heal your opponent but I wanted to check here as I found my players idea to be pretty good and wanted to make sure he could legally play his deck as we both see it to work.