I don't know if you'd call this ethical or moral or what but I think in a marriage you do take on some responsibility for your partners well being when they become incapable of ensuring it themselves. If you have a mentally infirm spouse you take on additional responsibilities for caring for them just as you would with supervising a child. You don't leave a gun accessible to your mentally ill spouse any more than you'd leave draino in the playpen. Legal liability - no, but as far as just doing the right thing, yes.
Insofar as the husband is concerned, I agree with you. What we don't know is to what extent he went to secure his firearms.
I'm certain, that if a national law was passed requiring gun owners who shared homes with people who have been treated for mental illness, or have been prescribed medication for anxiety/depression what have you, that mandated all firearms be moved out of the home...many THR members would be up in arms. (pun intended)