I'm honestly more concerned about your saddling her with fighter than any level discrepency. Fighter is incredibly swingy in terms of usefulness and ability depending on how its built. I'd rather strongly suggest one of the other fighter archetypes; barbarian, ranger, paladin, knight, hexblade, samurai, or swashbuckler. They're all much more stable in ability and aren't as prone to giving newbs option-overload. If complexity isn't an issue then fighter's okay, but you might also consider one of the martial adepts or a duskblade as they tend to be a bit flashier.

on topic: given that a healer can't actually attack the enemies, IIRC, the level difference shouldn't matter at all, especially if she never knows about it. ECL is a metagame construct that the characters shouldn't know anything about in-character.