Dating Advice!!

Victoria
Hey everyone. So my boyfriend, who is my first actual love, has this "best friend" who's an older female that he works with. Now, she is married but from personal experience I know that doesn't always matter. I'm not necessarily worried about him cheating on me, I just don't like how much attention he gives her. Tonight he wants to go over to his house and get drunk with her and her husband after work but he has class at 8 AM. He hasn't been going to class for weeks. I told him I felt uncomfortable with the situation and he got mad. Am I justified in being upset? Sorry this is long!