Taking care of elders is a challenge.  If you have never done it, you cannot imagine how difficult it is when you know an elder is not competent to live independently, yet refuses to move or accept significant help.  You have to take them to court to have them declared incompetent - so your choice is doing that (and making them so angry with you that the relationship ends) or just doing as best you can.  If you haven't been there, you should not judge.  If you have, you would understand.
Having a gun in any house is a risk.  Where there is mental illness the risk multiplies.   You have underscored my point - get the gun out of the house if you can.  If you don't have legal rights to sell it to a gun dealer, then store it offsite where no one can get to it easily.  Small storage lockers can be rented.  
OTOH who knows if anyone other than the man knew the gun was there?  I was hoping my comments would help consider getting rid of their guns before they did something they regretted.