Do you trust your girlfriend or her classmate? If you trust your girlfriend, don't trust that person at all. If you believe her classmates, go to your girlfriend to check the proof. If it's true, you can't stand it and break up.
But I still advise you, even if it is true, her past is over, so why care? Watson said to his wife, I didn't participate in your past, and I will accompany you to the end. Every woman will be moved.