Try various type of squats, body weight ones to help her get the hang of it. I think wall sits are an excellent way to help her adapt to squats.
I have some clients do squats in front of a stable chair or a bench.
I prefer a very stable bench, esp due to her weight. The stability of the bench with make her feel more safe and able to focus more on her form.
Remember to be sure the bench isn’t too low, you don’t want to put her at risk for knee injury. Since she’s overweight, she is already at risk for knee injury.
I have clients start by lightly tapping the bench with their bottom, to avoid her heels from lifting make sure she is far enough away from the bench and maintaining proper form.
As she gets stronger you can change it up…
You have some great suggestions here. One more think you could try is for her to do some bodyweight squats, but facing the wall. This is an exercise I have clients do when they first start with me and it’s also a great warm-up and stretching drill especially when doing kettlebells. She can start doing these squats about 12-20 inches from the wall and then gradually decrease the distance to wall (maybe after every 10-20 squats until she gets almost toes to the wall…this might take some time). It will be more helpful if she perform these without shoes and it will also help her with her flexibility.
I hope this helps.