In Dungeon Run, is there ever a reason to assist another player if you have full health?
Sure.  There are several reasons you might want to do this.  Obviously if there's a monster that will threaten you, and you're worried about it, and your opponent can't kill him on his own, you may want to assist.

On a more strategic level, you may want to cooperate, especially if there are more than 2 players.  See http://en.wikipedia.org/wiki/Tit_for_tat for a game theory perspective on building trust.  If you and another player are cooperating, you both get a benefit relative to the other players (as the next time he'll assist you, even if he doesn't have damage).