The Collaborative International Dictionary of English v.0.48:

pen-friend \pen-friend\ n.
   A person one comes to know by by frequent friendly
   correspondence; a pen pal.

   Syn: pen pal, pen-pal, penfriend.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

pen-friend
    n 1: a person you come to know by frequent friendly
         correspondence [syn: pen pal, pen-friend]