I'll be 57 soon, just bought a Giant Suede in May and absolutely love it. Haven't ridden since the early 90's but adapted very quickly with this bike. Love the comfortable handling but especially the pedals forward, able to put my feet down quickly aspect. Have had a couple of near falls but was able to recover because of the lower center of gravity and feet near ground position.

Go for it! Love all the time you can with your mom. She'll feel better. My back and neck (both problem areas) have improved with riding. My seatpost has suspension in it and does help with big bumps, ruts, etc. Be sure to have her wear gloves and a helmet. The handlebar is adjustable to ride very upright or lean forward more. Mine is very upright now but I might lower it soon now that my hands are used to riding.