Fly is right-- a letter like this really makes our day!
Make sure you also send the F/A a copy of the letter directly. In my experience, if you just address it to a supervisor, sometimes we find out about it; sometimes we don't. The best place to try it (unless you know the exact mailing address where that person is based) is to:
[F/A Name, File #]
Flight Attendant-- XXXSW
P.O. Box 66100
Chicago, IL 60666
That is the general UA mailbox systemwide. If you know where the F/A was based, use that three-letter code for XXX (i.e., ORDSW or SEASW, or wherever-- SW is the internal department code for Flight Attendant, left over from the old SteWardess days... ahh, tradition!).
Eventually, with a little luck, and based on the skill of the WHQ mail sorter, it will hopefully reach the person intended.