You mom seems to have had a long driving history, good for her. I hope she’s able to cope well with the loss of her driving. Is she willing to phone a cab if she wants to go shopping or something? I hope so, to keep the feeling of independence. Some folks seem to adapt rather well, and enjoy not having to drive, and instead just sit back and enjoy the company family, neighbors, and other elderly friends who live in their retirement place. I think that’s the main problem you have there, not the car.
For the car, suggest to slowly bring all the routine maintenance up to date. If you only drive 35 mph max on neighborhood roads, if the timing belt breaks the car will stall, won’t be able to start, so it will be an inconvenience, and may damage something beyond just the timing belt, but probably the only damage will be the broken timing belt. Suggest to not drive at higher speeds on the freeway until the timing belt is resolved.
Not just the timing belt, but all the rubber components are probably nearing their age limit. Serpentine belts, hoses, suspension bushings all need to be inspected. And especially worrying as a safety consideration at that age are the car’s tires. Make sure to get those inspected for any signs of cracking in the tread area or the sidewall. If the tires are original to your 17 year old car, time to start looking for some new ones.