someone who takes both sides in a diplomatic confrontation.
Two people get into a disagreement. A third person tries to make peace. The third person, the fence rider, will speak good or bad things towards each of the two keeping everything equal. See both sides of the argument. Sometimes used in a negative sense, meaning that you can not depend on this person to support your argument.
He'll never take sides, he's a fence rider!
Traitor, Coward, Snitch
Someone who appears to take your side in an argument, then supports the opposing side. Could also be considered a "Traitor", because he may give privileged inside information learned due to his preceived allegiance to the opposing side. (Word comes from changing the appears of your uniform or turning your coat inside out to appear to be on a different side.)
That "turn-coat" told them our strategy to win in court.
Be sure to not let the "turn-coat" overhear our plan of attack.
Did you see that "turn-coat" run when the fight started getting rough?