It belongs to A as he was helping B. However, clearly given the time involved between the shot and finding it bedded A did not make a vital hit. If A had shot it and it ran onto another property, and hunter C shot it while it was still on its feet, C would have every right to claim it.
I don't believe in "first blood". If you can't shoot good enough to put it down the first time you lose your "rights" to it.
One year I saw a buck run through the woods in front of me but could not get a shot. Five minutes later I heard a shot 1/4 mile away on the neighbor's, figured he'd shot it. Ten minutes later I see the buck coming back up the woods on the other side of me (it had run in a very long loop around me). I got up and stalked it, found it bedded down and shot it. I did not notice that it had indeed already been shot so I tagged it and left it lay to go get the UTV. Well I'd been back up at the farm for 30min getting the gear together to retrieve it and the neighbor shows up, mad as hell that I'd tagged HIS deer (he'd tracked in onto our property WITHOUT our permission and located it with my tag on it). Turns out he shot it in the butt from 30 yards. I told him if the best he could do at 30 yards was shoot it in the butt, he had no right to the deer.
Once I had a hangfire with my muzzleloader and made a bad shot on a buck, hit him in the leg. I tracked him for 3-4 miles before I finally put him down, passing a dozen empty deer stands. I was HOPING someone would put him down, to end it. I would have congratulated them and told them it was their deer, after a bad shot I did not deserve it.