My friend eats food in secret. is it an disorder?

Maybe: Eating food in secret is more likely to represent a disorder if the person is too heavy or too thin and it is also affecting his every day activities....

Could be: If it isnt one yet- it sounds like its on the way to becoming one. I would be concerned about bulimia. They would be the ones to hoard food in private and vomit. It might be time to see a psychologist and find out why she feels the need to behave as she is doing so. Catch it early before it become a serious problem.