What are the causes of leg/calf muscle cramps?

I have a friend who is 36, on the urge of being diabetic, taking medicine to help so they she doesn't become diabetic. Every time she walks, or runs a lil her legs cramp up. A guy at a health food store said she is lacking protein and needs to try a protein powder to start off with. Then try a multivitamin. He said that she didn't have to take a multivitamin forever just until she doesn't have cramps anymore. I remember from watching doctor oz. that you should always take a multivitamin. What could it be?