Nothing wrong with them owning a firearm. But in their case it's not going to help them. If they're scared of it and keep it locked away it won't be ready when they need it. A break in or attack happens right now, not in a minute or three.
You've had some good feedback about alternative security measures. To those I'd add Tire Billys and short machetes. I have a few of each strategically located around where I might be if a break in happens.
I, too, would question their mindset to employ lethal force even if they manage to get a firearm into play.
Good luck.