Remove Device from Market Account? Hear me out. I know that this has been asked before, but I am hoping that there might be a workaround or some new information.

My boyfriend has a [Phone X] that has been sent in for replacement twice, and he now has two [Phone X] registered to his google account, and neither one of them is his newest phone. 

This is the weird part: somebody has one of his old phones and is using it. He can see everything they download and install onto the phone. 

What is going ON here? I know it is possible to "hide" that phone, but it just does not seem good enough, from a security standpoint.  His android market account is connected to a google wallet account. 

((I am the one asking because I am more interested in tech and more patient about troubleshooting.))
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.