Have you seen her "out and about" ? Is she really mobile? At that weight I would think stairs would be a strain on her heart. Have you mentioned that your sewing room is down stairs?
Do not be afraid to bring the issue of her mobility up... she is more than aware of her weight and the restirctions. Be kind and have an concerned discusion , ask her how to best facilitate the leaning process, so she can particpate. In your conversations decribe all that is entailed , but in a lesson plan kinda of way.
Most people who have this type of issue will typically respond well if you ask frankly and with genuine concern.