No he wasn't there. FBI agent would not be allowed to participate in a murder
based on what informants said at Massino's trial, there were backup guys a few blocks away who did not know the details of what would happen and sat waiting in case called upon. One car contained "Lefty and a few of Sonny Black's men".
it's possible that Lefty told Pistone to come along which meant the FBI would not have known in time.