He needs to do it at least twice to prove anything. If we were able to pick and choose our best groups ever, we'd all be marksmen.
One three shot or one even six shot group proves nothing. You can sometimes pair a gun that shoots all over the place with a shooter who's the same, and come up with what your friend produced. Granted with six shots the likelihood of the above is very small, but it's only one group.
I use a minimum of two, five-shot groups to establish how my gun-shooter combinations shoot.