I'd say that if he is actually your buddy, then you both lay claim to it and are tickled pink.

If he's being a prick about it, then it's technically his deer.

I know two brothers that were bow hunting about 100yds apart, one shot a p&y 10pt, and it ran over in front of the other, and he shot it as well. Deer piled up about 30yds later. Both shots were through the lungs. They have a nice deer and a cool story about the buck "they" killed.