Sounds like your "so called" healer either, forgot to untick the healer option in GF, in which case he's a moron or deliberatly left it ticked to get a group faster, which makes him both a moron and a jerk.
Or, as in many cases, the group finder reset itself and checked healer for him.
I've seen this happen many times on my Sorc and on my Merc. I can group find with only DPS checked for days. Then, all of a sudden, Healer will be checked for both of them. If I don't catch it, I'll end up joining a group as a healer.