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…