First, check and make sure it is not a private or hidden group (I assume not since other users can see it.)
Then, try making a copy of the user account, have the user log into it, and see if anything's different. (To make a copy, go to where you would create a new user, but instead of typing a user name, pull down and find the user you want to copy. It will come up as "Copy of...")
I have seen it where user accounts get corrupted and need to be recreated. It is rare.