The Collaborative International Dictionary of English v.0.48:

Abetter \A*bet"ter\, Abettor \A*bet*tor\, n.
   One who abets; an instigator of an offense or an offender.
   [1913 Webster]

   Note: The form abettor is the legal term and also in general
         use.
         [1913 Webster]

   Syn: Abettor, Accessory, Accomplice.

   Usage: These words denote different degrees of complicity in
          some deed or crime. An abettor is one who incites or
          encourages to the act, without sharing in its
          performance. An accessory supposes a principal
          offender. One who is neither the chief actor in an
          offense, nor present at its performance, but accedes
          to or becomes involved in its guilt, either by some
          previous or subsequent act, as of instigating,
          encouraging, aiding, or concealing, etc., is an
          accessory. An accomplice is one who participates in
          the commission of an offense, whether as principal or
          accessory. Thus in treason, there are no abettors or
          accessories, but all are held to be principals or
          accomplices.
          [1913 Webster]

WordNet (r) 3.0 (2006):

abetter
    n 1: one who helps or encourages or incites another [syn:
         abettor, abetter]