You’ve received sound advice and what I say isn’t meant to contradict it.
I would back up a step and see what types of self care she doing and whether she’s been to the doctor for her plantar fascitis. It can vary in severity. I have battled it for years and when I’m not taking good care of my lower legs and feet, it acts up.
If I were training her, I’d get her on a program of calf and peroneal stretching, arch strengthening, anterior tib strengthening, and ankle flexibility in all three planes of motion, as preparation for ambulatory cardio.
But, as Jane mentioned, there are plenty of ways to challenge one’s cardio system without doing the traditional walking, jogging, etc. that tends to have a lot of force on the arches. Get the big muscles like glutes and quads moving, and you’ve got heart-rate as those muscles demand oxygen. Even a weight training circuit, with compound exercises, can get a person into target heart rate if you alternate muscle groups and keep them moving rather than resting between sets.