Remove Device from Market Account?
First, I wouldn't be that worried about 'hiding' the phone. I would be worried that if he can see what they download, his account is still attached to that phone. which means they can see EVERYTHING he downloads, emails, chats, etc. 

He needs to change the password on the gmail account. Maybe, for extra security, turn on 2 step verification.

You can't, at this time, remove a device from the market. What you can do is go to your Play settings and you can hide the device so they don't show up in the menus.