When I was working in geriatrics I was usually in the physical therapy department as a restorative aide. When I was walking them, of course, they were using a walker but they always wore a stiff soled shoe. The ones we would use were called: Orthofeet. They were for men & women but extremely expensive. After they were put to bed for the night, depending on how bad it had gotten, we would use a velcro device that wrapped around each leg and down to the arch of the foot supporting the correct 90° angle. Sometimes we'd have to put a triangular pillow at the foot of the bed.
I hope this helps to shed a little light on what you're looking for. Oh & also another thing I'd make sure to tell them every time I was walking them was to say: Heel-to-toe, because I was watching their foot placement with every step they took.
Much love,
Ashley Faith