How deep does she go? How wide a stance? How heavy is she lifting?
She does not go below parallel. Our thought was that if she can stand up from a chair without any pain then what would happen if we put weight on her back? We started out with a chair and now she uses my weight bench. We have been doing this for a few months now and started very slowly. She is now up to 70 lbs. doing 3x5. Her stance is wider than shoulder width. She is doing the low bar version. Doing high bar does hurt her knees.
She does not use any knee supports yet. But I am sure that will change eventually. Also paying very close attention to pushing your knees out when lifting is very important. DO NOT let them buckle inwards. If we start running into problems with knees trying to buckle inwards we will lower the weight and raise the reps. until we hit a goal amount of reps with perfect form.
When lifting from the seated position make sure to lead with your chest up. This causes you to lift at your heals and not your toes. That is very important as well. This puts less pressure on the knees.
Again, I am not an expert on this. We have just found that this works for us.