It says on this link that you can give your piggy green beans a couple of times per week.
They have a wonderful chart to let you know when, and what, and how much fresh veggies to give your piggy.
Here's the link:
Guinea Pig Cages Forum
Then go down the list to "DIET AND NUTRITION"
Then go down the list again to find, "EVERYTHING YOU NEED TO KNOW ABOUT A CAVY'S DIET!!