We did this last week. IF the campanion travels using the "certificate" they must travel in the same class and same flights as the other full paying passenger. IF they do not travel they just drop the non-flying pax from the ressie. So if no outbound is flown, no return is available for the campanion either. They have no problem with this because AS gets full fare for one pax and an empty seat to potentially fill and you can't use your certificate again. They just don't want to campanion trying to go onto another flight or upgrade by themselves but then again anything is possible at the gate depending on space.