Yeah, pretty much, if he wants to let the kid do something that's NOT illegal, it's his right to do so. Be it getting a purple mowhawk, taking him sky diving, taking him to a different church, giving him different POV on political issues, talking to him about sex, drugs, alcohol, and smoking; or teaching the boy to shoot a .22.
If momma don't like it, TDB!
You obviously don't get it and don't seem to have any intention of trying to.
Just because two people are no longer married, doesn't mean they don't have a moral responsibility to communicate with each other about the upbringing of their child(ren). Fortunately for me, after my divorce the ex and I were able to work togther when it came to our son. Even though he lived with me 1,500 miles away from her, we still talked about things that directly concerned him, or at least he would run things past her before just going out and doing them. It all depended on what the subject and the importance of that subject was.
Before he got his green Mohawk at the age of 13, she knew about it. Before his ear was pierced at the age of 16, she knew about it. Hell, before he got his first tatoo at the age of 18, she knew about it. By his choice, as a man.
Unfortunately, guns in our society are a touchy issue. Not everyone is enlightened and believes in the Second Ammendment. More importantly, if this woman is looking for a way to cause problems, this might very well be the way to do so.
As he said, the kid knows his mom and he's on the QT with the gun info. There's probably a good reason for that. And if she ever finds out, then she has two beefs when she goes to the judge. #1, he didn't tell her he was carrying around the kid and #2, the kid knew so he was having him keep secrets from her. My understanding is that family court doesn't like that.
His best bet, as expensive as it may be, is to contact a lawyer that specializes in family matters and see what they think. That way he can make an informed decision based on the law. Alternately, he should just tell her and see what happens. She might be cool with it. What he should not do under any circumstances is listen to the uninformed opinion of someone like you.
Have a nice day.