This is a tough one.
First of all, I doubt the restaurant's story. She would "...pile up her plate, take one bite, and then throw the rest away"? That doesn't sound right. Maybe if the food was bad...but then she goes back and gets more?
Something sounds fishy...
Bottom line: the restaurant didn't like her cutting into their bottom line. They should have asked her (politely) to monitor her habits. It's a buffet, after all; if you don't like food being thrown away, kill the buffet and only offer entrees.