How can I talk to my boyfriend about his friend being a bad influence?

I have been dating my boyfriend for almost two years now and he has a friend that he has known for a few years now. When I first met his friend, we all hung out just fine. But, my boyfriend’s friend has been known to hang around some bad people. I had asked my boyfriend’s friend to please not bring my boyfriend around a friend he likes to buy marijuana off of. My boyfriend’s friend still brought him around his dealer. This really upset me. Now, I know I cannot control my boyfriend and his life but what makes me mad is that his friend had specifically told me he would not bring my boyfriend around his dealer. My boyfriend’s friend and I didn’t speak for awhile but then I started to realize that when my boyfriend and I would fight, he would go to his friend’s house and act differently when he came home. I started to talk to my boyfriend about how I felt and what was going on. From how he explained how he felt, it sounded like something his friend would say. Now, our fights have gotten more frequent and my boyfriend has become hateful and distant. I feel as if my boyfriend’s friend is influencing my boyfriend’s opinion and feelings in our relationship and its slowly breaking us apart. How do I bring up this concern to my boyfriend? I already have gone out of my way to be civil with his friend and illegitimately apologized to him.