This question came up at a customer and while there could be a few ways to do this, the following is one of the ways to do this.
The TF checkin command has an option called /author. Using the author option one could check in another user’s changes. This is useful in situations such as when the user checked out the file, made some changes and then left the organization without checking it back in.
The format of the command is:
tf checkin [/author:author name] [/comment:("comment"|@comment file)] [/noprompt] [/notes:(“Note Name”=”note text”|@notefile)] [/override:reasonfile|@reason] [/recursive] [/saved] [/validate] [itemspec]
Ofcourse to do this the user using the above command must have the “Check in other user's changes permission” set to Allow.
The full details can be found in http://msdn.microsoft.com/en-us/library/c327ca1z.aspx