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?