Removing Modem from my account that isn't mine

My apartment has two modems, one downstairs and one up, each with a separate account through Xfinity. The lower modem was from Comcast and was already active on my roommate's account for a year before I bought a Netgear modem and Xfinity plan for myself upstairs. However, during the activation process I inadvertantly activated HIS router on my account due to being plugged into it via Ethernet, instead of plugging into my own new modem and activiating THAT. As a result, both my own modem and his are on my account and he cannot get his modem back on his account.

I'm hoping Tech Support will be able to help me, but I'm also wondering how is this even happened. 

How can you steal a modem from an already active account and put it on another? Isn't the modem's MAC address assigned to the one account?