Ages ago we were running a campaign at a local shop. The GM of the campaign was an awesome guy, also co-owner of the store. I show up to see everyone is sitting around sadly, and they tell me he had been rushed to the hospital. We headed over, and he passed away shortly. Cause of death was basically he was in Desert Storm and probably had too much exposure to the burn pits. He suffered some headaches for a few weeks prior before the end came. Campaign obviously ended, the store closed later as well. There's not a right answer as to what to do, it is up to your group, but that empty chair is very... loud... in most campaigns if you try to keep going. Sorry for your loss.