You can try covering her with a towel, but frankly it's almost possible to keep your mom covered with a towel for the whole bath. Try this approach: wrap a towel around her waist if she's standing, or place it over her lap if she's sitting. She can even put the towel around her waist under her bathrobe before the bath and keep it on the whole time, though this can be a little tricky when she's getting in and out of the shower or tub. Lift the towel to wash under it as needed, a small section at a time. With a little practice you can do this quickly without exposing much of her body.

If this still seems to be too much for her, you can actually wash your mom through a towel without lifting it. Soap the cloth into a lather, massage it into her skin through the fabric, and rinse with fresh water. This works for the lap, breasts, and buttocks, but you'll need to reach under for the privates and backside. All of these ideas work for a man as well.