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.
I would structure your circuit, alternating upper body and lower body exercises. Which will help to keep the heart rate up. Of course keeping the lower body exercises appropriate for planter. Maybe mat work, side lying leg raises, clam shell, mule kicks, fire hydrants, all come to mind first. Cardio wise, why not think a little unconventional, have her hold v-sits and toss a med, v-sit and cross punch, the move I call, the dead bug (lying on your back, or v-sit position, arms and legs straight and “scissor ” arms and legs). There are several other multi-jointed or combo movements you could up with that would help with the heart rate.
Hope you find this helpful!
I often find that plantar fasciitis prevents ANY cardiovascular exercise where the feet are on the ground. A bike or upper body ergometer are often the only options.
I would look at different modalities such as peripheral heart action system and create a circuit that keeps the heart rate elevated throughout without the need for cardio that exacerbates the plantar fasciitis.
I would also suggest that you look at modalities that help with plantar fasciitis such as the MELT Method Foot Treatment or the approach that Justin Price is taking https://www.youtube.com/watch?v=gqIYKs0YUcs.