I don't think I have an obligation to tell someone's girlfriend that he cheated on her, so I'm keeping it to myself.
She might want to know though. Why don't you want to tell her?
I think this news is going to hurt her a lot
You don't have to involve yourself in other people's relationships. Sometimes its best to stay out of them.
She's my good friend and I want to avoid pain for her.
It's a bit different if this is your good friend. You may want to consider telling her, so she doesn't continue the relationship only to find out much later. That will hurt her more than hearing it from you now.