If there is a rule violation he will receive the notice from the FCC. He can then respond with the call of the person who was operating as control operator of that position at the time the violation occurred.
But he can't just pawn it off on the other guy. The holder of the callsign that was being used is just as responsible for the operation of the station as the actual control operator. While I suppose technically the licensee does not actually have to be present, since it is his license on the line, it is probably a good idea that he is present.
§97.103 Station licensee responsibilities.
(a) The station licensee is responsible for the proper operation of the station in accordance with the FCC Rules. When the control operator is a different amateur operator than the station licensee, both persons are equally responsible for proper operation of the station.