Shoutbox

If Messenger is logging on as his account, you will need to remove it from your Managed Networked Passports.
Assuming you are running Windows XP you can do this by clicking Start > Control Panel > User Accounts > User Accounts (Again).

In the User Accounts window, click your appropriate User Name and then in the top right hand corner within the border under related tasks should be "Manage my Network Passwords".
Click this...A new window should pop-up, if your friends account is listed here. Delete it, it shall remove it from your stored list. Also...There might be one that is called something along the lines of "*Passport" (Passport with some symbols). Delete this also, this is the automatic login account. If you have set your account to automatically login, delete the "Passport" account anyways, once back into messenger you can tick "Remember my login" if you desire to.

I think this should stop your friends account from logging in
(I think thats what was happening, sorry if it wasn't)
If Stigmata's idea doesn't work, try this as a back-up.

Hyped