PLEASE ASWER! how to explain to a guy that what he is doing to his gf is wrong?

Tell him that if he can't keep himself with just her, that it's not fair to her. Ask him if he would be mad if he found out that this girlfriend was sending photos to other men. And they were sending photos back. If that still doesn't faze him then it just means he has a completely different perspective of what he thinks is right and wrong in a relationship and tell the girlfriend to leave him because he will never stop doing this.