Tell her this: Older people are natural targets for bad guys. When you are in your 20's, 30's and 40's you can react and move quickly, and look like you can fight off (or at least cause trouble for) your average thug. As you get older the "BEAT & CARJACK ME" sign on your back gets bigger and bolder every year. Where you might be able to avoid wearing a 10 karat diamond on your finger, you can't avoid getting older and (perceptibly) weaker. If you are going to protect yourself in your older years, you will need a firearm and you will need to learn to be proficient with it while you are younger and capable of getting the proper training.
When an older person gets to the point in their lives where they need to use a cane, wheel chair or other device to assist them to get around, nobody questions that. Well, a firearm is just such a device - you can choose to be a victim or choose to learn a "new" way of providing for your own protection.