A rigid frame comfort hybrid cruiser should be comfortable. Don't over fill the tires. Most large tires for comfort cruisers can be run around 30-50 psi, depending on the tire, rider weight, etc. Tires are often marked something like 75-90 psi but it's rarely necessary or desirable to run them at full pressure.
There are very few folding bikes that can be as comfortable as a full sized bike. The shorter wheelbase prevents that. There are a few larger folding bikes. But if the emphasis is on riding, rather than storage, get a regular bike. Folding and unfolding a bike for every use will eventually be a deterrent to riding at all and eventually it'll gather dust before it gets old and gets sold.
If you do get a folding bike be sure it unfolds and folds as simply as possible. There are huge design differences. Check the folding bikes forum for tips. For example the Strida bike may be the easiest to fold and unfold, but it's not necessarily the easiest or most pleasant to ride. The Brompton may be the most practical folder, but pricey. Ditto, Bike Friday and a few other premium folders. Most of the affordable folders in the $100-$200 range are badly compromised and may discourage your mom from using them more than a few times.
When I resumed cycling in 2015 I was recovering from a broken neck and back from a 2001 car wreck. I needed a cane for longer walks until 2014. So I started with a comfort hybrid, an early to mid 2000s Globe Carmel. Upright ride, simple spring front fork, long wheelbase, thickly padded saddle with spring suspension. It helped at first. But the main difference was tire pressure. Getting better, fatter tires and running them around 50 psi really helped.
I still do that on my rigid frame/fork hybrid. I use Continental Speed Rides, a great value in hybrid tires, and run 'em around 40 psi front, 50-60 psi rear depending on whether I'm hauling groceries (heavier weight needs higher tire pressure).
Same with my road bikes. I usually set my skinnier 700x23 and 700x25 tires to around 85 psi rear, 65-70 psi front -- much lower than the sidewall specs suggest. At my weight (around 150 lbs) that's appropriate for me. Much more comfortable on chipseal and rough pavement.
And encourage your mom to do physical therapy. Due to the pandemic it'll mostly be self directed at home, which is harder. Check out the Bowflex channel on YouTube for some very good videos of 3-5 minutes on exercises that don't require any special equipment.
It's hard, I know. Been there. I was hit by a car again in 2018, breaking and dislocating my shoulder. Took about a year to recover. But I do some PT every day, even if it's just stretching and range of motion.
Pain from working stiff muscles and joints is a powerful deterrent, but it works. I don't want to end up like some of my family and neighbors, who were practically immobile by their 60s, unable to get up off the floor or couch without help. I was in nursing for years and was primary caregiver for three older family members. I don't want to end up like that if I can prevent it by doing some basic exercises now to regain strength and mobility.