Sad to say, but yes. This kind of behaviour when a player decides to join mid-match as second gunner is tragically becoming all too acceptable.
Howard has assured us that Quick Start won't drop a second gunner onto a crew, so the player must be doing this deliberately.
(I'm more restrained in my language when this happens, but I will very strongly encourage a player to exit the match, start-stop a Sandbox game, change role to engineer and come back)