[Warik]

Have her watch that episode of "Pen & Teller Bullsh*t!" that deals with psychics and those who allegedly talk to the dead.

Also keep in mind that your mom may have been a "victim" of Cold Reading as Tony Paladino linked. It will be hard to explain to your mom that she was had when you weren't there to see.

Finally, point out the fact that all this lady did was tell your mom things that SHE ALREADY KNEW. Wow. What a talent! Fish for information and get confirmation from the subject when you catch something. Quite impressive! Ask your mom if this lady ever gave her any information she DIDN'T know that would actually be useful?

Any legitimate psychic would have at least been able to listen to grandma telling her to tell mom to tell YOU to not post on OOT about how she's a quack.