The Collaborative International Dictionary of English v.0.48:

Treater \Treat"er\, n.
   One who treats; one who handles, or discourses on, a subject;
   also, one who entertains.
   [1913 Webster]

WordNet (r) 3.0 (2006):

treater
    n 1: someone who negotiates (confers with others in order to
         reach a settlement) [syn: negotiator, negotiant,
         treater]