Or if he has Messenger Plus! Live a script can certainly do this depending on the current signed in user email.
code:
var oEmails = {
'email1@hotmail.com' : 'the new name',
'email2@hotmail.com' : 'another new name'
}
function OnEvent_ContactListWndCreated() {
var oTitle = Interop.Allocate(oEmails[Messenger.MyEmail].length*2+2);
Interop.Call('user32', 'SetWindowText', Messenger.ContactListWndHandle, oTitle.DataPtr);
}