You seem to have accepted the fact that she's going to keep it; that's good. She's your mother, and she's 70. She's not going to listen to you. She potty trained you, and if you make her, she WILL bring that up. So, you need to vent, you might be hopng for a gem of advice, but probably just a healthy bit of comiseration. Grown men need that, too.
Man, my mother doesn't have a gun, of which I'm glad. Some people shouldn't have them. She's one. My mother still insists on driving. It's not so much that she's getting too old to drive (though maybe that, too) as that she's been a menace to public safety and order in a motor vehicle her whole adult life. Some of my earliest memories involve lying flat on my back in the back end of a Buick station wagon, pirouetting and generally renacting Disney On Ice all over Abeleine. Being removed from a destroyed Geo Metro through the rear windscreen on a board by firefighters hasn't persuaded her to put away the keys, so I'm pretty sure I have no hope.
What you have going for you is that it sounds like, if you left her alone about it, your mother wouldn't go to the range by herself. She's not likely to TRY to shoot it. If you're really concerned she'll hurt herself, or that an intruder could take it from her, and all loving appeals to reason have failed, you might offer to 'clean' it for her, and grind down the firing pin. At least then noone's going to get hurt with it, and if you don't take her shooting, she'll never know. Alternatively, if she goes shooting and it won't fire, maybe you can talk her into buying something 'more reliable' instead of 'throwing good money after bad' by 'trying to have it fixed'.