Should I Diss My Best Guy Friend?

So my best guy friend has lately been dissing me for his "girlfriend". She's not actually his girlfriend, but a girl who is cheating on her boyfriend with my best friend. Anyways, when I need him the most he will simply cancel because something with her came up. At times its nothing serious, like one time I really needed him to help me with my french oral which was due the next day and I had asked him ahead of time if he could help and he said that he could. At the last minute his "girlfriend" call him to have a romantic dinner and he totally cancel on me. He's done this to me on several occasions to me and I'm starting to get tired of it. I've tried talking to him about it and he doesn't think what he is doing is wrong. Not to sound jealous or anything but sometimes but I dont understand why would he diss his best friend for a girl who wont leave her boyfriend for him. I don't want to be a person who he comes to when his "girlfriend" disses him. Should I just diss him altogether?