The Collaborative International Dictionary of English v.0.48:

Frequenter \Fre*quent*er\, n.
   One who frequents; one who often visits, or resorts to
   customarily.
   [1913 Webster]

WordNet (r) 3.0 (2006):

frequenter
    n 1: a regular customer [syn: patron, frequenter]

Moby Thesaurus II by Grady Ward, 1.0:

20 Moby Thesaurus words for "frequenter":
   aficionado, attender, audience, buff, caller, company, denizen,
   fan, freeloader, gate-crasher, guest, habitue, haunter, moocher,
   patron, spectator, theatergoer, uninvited guest, visitant,
   visitor