You could be selective about when you get together (the easy solution) - or you could make it your personal challenge to find out why your friend likes this doofus. Lastly, be sure to check your feelings - you could just be jealous.
I know how you feel. I felt the same way a few years back. My bff and her new guy were always together. It was hard to find girl time with her after she starting seeing him. Maybe you will luck out like me. She finally gave him the boot. Our friendship survived because I never told her how I felt. I truly didn't think it was any of my business. It would have hurt her feelings because she had stars in her eyes at first, while mine were rolling back in my head! What would telling her my feelings accomplish? I would only be telling her she was stupid and made bad choices. I say bite your tongue and give it time. Don't put all your eggs in one basket. I know how really hard it is to make friends, but expand your social life. You could also find a new hobby or interest. Your friend will value your acceptance. It will strengthen your bond. He will be long gone at some point and your friendship will outlive him.