My girlfriend's classmate is playing with her belly button, what can I do?

What I'm telling you is true and I would really like some advice on what I should do, thank you.
My girlfriend studies medicine, and one day her class was working on appendicitis (a disease around the navel) and it was a pair work, so she paired up with her famous classmate (she didn't know him yet at that time, it was really a coincidence). Then that's where it all started, she often wears clothes that expose her stomach and her navel and one day while they were working together, he offered to examine her navel to see if she was does not have the disease. She told me all this, but I know much more thanks to people I know in this class. When he supposedly had to examine my girlfriend's navel, he took the opportunity to tickle it, to play with it and I was told that he even licked it a few times. And she just laughed at that. Once he even came to my house to study with my girlfriend while I was away, but I came home a little earlier than expected and I caught the guy quickly rotating his finger in my girlfriend's navel. At first I thought it was an attack because he had overpowered my girlfriend to play with her navel, completely helpless, but she explained to me that it was a game between them. From now on she's the one who's going to study at his place, but I can very well imagine what's going on there.