Air carrier ground personnel reported during push back the aircraft rolled forward after the tow bar was disconnected. Pilot took evasive action and applied the brakes.

Narrative

I was training an employee; during pushback training trainee asked captain to set brakes; captain said brakes are set. Disconnect person tried to disconnect pressure between pushback and towbar. Disconnect person attempted to disconnect towbar from aircraft but was unable. At that point; we noticed the aircraft was rolling (still connected). I asked pushback driver in training if the brakes had been set; trainee confirmed that pilot gave the communication.I got within eyesight of the captain; gave him the brakes set symbol (clenched fists). He returned the symbol and gave a thumbs up. I returned to the towbar and disconnected from the aircraft; proceeded to ask the trainee in the pushback to back up. The aircraft then began to roll forward; as the brakes were not in fact set. I told the pushback driver and the employee disconnecting the towbar to back away from the rolling plane. I immediately returned to the pilot's view and gave the clenched-fist brakes set symbol again. The pilot looked down towards the brake pedals; and gave a thumbs up; stopping the airplane. I then entered the pushback; and we crew all returned to the gate.