Free: Contests & Raffles.
No offense to you but if that deer was broadside and hit behind the shoulder at a proper height we would be looking at pictures and congratulating you guys. My thought is either the deer was quartered towards him and entered the guts or possibly a clean miss under the deers belly and the arrow is buried in the dirt. Did you see the arrow in the deer as it ran off? Thick country at all? If it was indeed a double lung hit that deer is most likely dead within 100yards of the shot. Probably preaching to the choir but quartering angles are a much bigger deal with a bow than with either a rifle or muzzy.
It sucks but it happens with every style of hunting eventually. Best thing to do is get him back on the horse as soon as you can.