How good a friend are you? If you guys are close, I would politely tell him fiends don't let friends buy guns and not teach them safe gun handling.
I think it will take more than a few rips to the range. NRA safety course offered in your area? Does the gun store have instructors available?
I would politely insist that both he and his wife attend and let them know that you care enough to not want to go to one of their funerals because you didn't make the effort.
If they refuse, only then would I advise them that they shouldn't own a gun.
Not only do you have the right, you have the moral obligation.