If she is a big fan than let her.... she knows the risks involved and there are probably rules for the FA that either allow it or dont, my guess is they dont, but she knows em and she accepted the risk. As for the passenger's sake, if its a big enough celeb, they are used to it, regardless of on a plane, in a restaurant, or walking down the sidewalk. If it bugs him/her, then let them/agent report it to DL.