Someone else will be given the seat if it is needed. You could cut down on the chances that the seat will be given away by checking in for the seat and having your companion no show at the airport. That may inconvenience BA a bit if they think your companion is at the airport and try to have him paged, look for him, etc. In reality, when he doesn't board at T-15 min or so, he'll be assumed to be a no show and his seat made available to the GA for someone else. It'll depend on how full the flight is.
Safe Travels.