The Collaborative International Dictionary of English v.0.48:

Excuser \Ex*cus"er\, n.
   1. One who offers excuses or pleads in extenuation of the
      fault of another. --Swift.
      [1913 Webster]

   2. One who excuses or forgives another. --Shelton.
      [1913 Webster]

WordNet (r) 3.0 (2006):

excuser
    n 1: a person who pardons or forgives or excuses a fault or
         offense [syn: pardoner, forgiver, excuser]