I don't think they are susceptable to it... I't susually reserved for discus and some of the larger cichlids. Then again, I think they're fairly closely related to discus so it wouldn't really surprise me. I believe it isn't something that would develop that quickly, or on a lip though. Not really sure. From what I remember, it comes from either poor water quality or poor diet, or more likely a combination of these.
Is there anything sharp in the tank that he might have banged it on?