She'll be fine. Aside from Machka's comments, my biggest concern would be that since she's never been on a bike for longer periods of time and is accustomed to workouts designed to maximize output, she might pedal too hard and tire herself out prematurely. What kind of bike is she riding?
If she's on a hybrid or MTB and you're on a road bike, you might want to take an especially easy pace. Many 23 year olds will not want to admit that they're having trouble keeping up with a 59 year old, so if your pace looks anything other than easy for her, you might want to slow down a bit.