Restrict SQL Server 2008 R2 User to restore only one database (but not others)
Log in as a server admin.
Create a stored procedure with execute as owner that restores a particular database.
Assign permission to execute it to the user.

As for where to put that stored procedure, I'm not sure it's possible to restore the DB from which you are running the SP, in which case you might want to create a completely blank database for the only purpose of storing this stored procedure.