Sounds to me like a lawsuit may be brewing. What she is going to argue is that she paid for one, so she should get that one. What he'll argue is that he was in ownership of them both and they he should decide who gets them (obviously, the topic of your post).
I would say that she may have legitimate claim over the one she purchased, however, it's not like there are any receipts for either. You feel that if she gets the guns, she'll hand them right to him, which you could argue in court, but you'd have to show that this would be the case. What she needs to be aware of is that if she knowingly gives the firearms to a person that cannot legally possess a firearm (as I see it, it's no different than handing it to a random child on the street), then she may be committing a crime herself.
If it were me, I would say keep the guns and tell her that your son is not to have anything to do with firearms any more, and she isn't going to get them because all she's going to do is harbor his problems. If they really want them back, let them get a lawyer and spend their money first before getting your own. Chances are, they won't do anything about it.
"The Constitution is not an instrument for the government to restrain the people, it is an instrument for the people to restrain the government - lest it come to dominate our lives and interests.” - Patrick Henry