It's hard to explain really but very simple when you think about it. I'll attack this in reverse order. We all know how important photoperiodism is to a deer and it's behavior. We know that after the Second Fall Equinox or the 'rutting moon' is what really triggers the rut. After the Summer Solstice (June 21) the days begin to gradually get shorter. As the days shorten the pituitary gland produces a hormone that that enlarges the bucks testicles and causes them to descend. And by the beginning of August they begin to produce sperm. Bucks are unable to breed from generally from the end of April to the end of July. The amount of sperm peaks in November (the rut) and drops rapidly by mid-December (the rut's waning days). And then process of the testicles receeding slowly begins. It doesn't happen overnight and as you can see is a very long possible. Chances are the buck that your friend saw breeding a doe obviously still had enough to breed.