Chicks like healers usually. At least from my experience that's what they seem to play most.
I agree, tank is the most girlfriend friendly class if she is new to MMOs...
I've known almost a dozen female MMO players. Over half were healers. None were tanks. They don't seem to like tanking.
Edit: I take that back. I knew one with a tank alt in Lineage 2, but my point stands. Tanks are not a popular class for female gamers.