How can I protect my 84 year old mother from the "snake oil" salesman that is draining all of her money?

You could check the Better Business Bureau and see if there are complaints against this sales company. If there are, that may help her understand that she is being taken for a ride. There are good supplements around, but she doesn't need to be spending hundreds of dollars to a direct sales person. She does, however, have rights, and if she wants to spend her money this way, and she isn't cognitively impaired, it really is her choice. I know that's frustrating, but unless this person is crooked, I'm not sure you can do much.
Take care,
Carol