Is he left handed?
I'm a south paw and when I snap the release i consistently shoot left.
I'm going to guess it's a shooting form issue.
Also, does he have the same draw length to you? If not, that will cause form issues.
Might want to tell him to take the bow to your local pro shop to get fitted.
Also, shameless plug time- we're recording a podcast episode tonight on this very subject. Should be available to download in a couple of days.
Cheers.
John
www.highlanderhunting.podbean.com