I read about this a while ago, so my explanation may be off a little.

The scope isn't mounted straight. There is a slight imperfection which is based upon the way your friend holds the rifle. It could be where he mounts his cheek, or how he grips the stock. Whatever. Anyway, he shoots well at 100 yards because it is sighted in there. At longer distances, the problem should be more pronounced for him.

For you, the problem is well pronounced at 100 yards because you hold the rifle altogether differently.

I hope I've said this right. I bet there are guys here who will point out errors I've made, or whatever, but I think I got the jist of it.