What's a good excuse to get out of going to a party you don't want to go to?

My boyfriend's friend invited us to her birthday dinner the other day. Me and the friend don't get along and she knows i don't like her. Every time she goes out she gets wasted and acts like a total slut and needs to make herself the center of attention. It would be fine enough for me to just say i don't want to go but that doesn't excuse my boyfriend from going. I told him he can go if he wants but he said he doesn't want to but wants to make up an excuse because he doesn't want to hurt her feelings rather than bluntly telling her he doesn't want to go. does anyone have any good ideas? we haven't had to give an answer yet because she hasn't told us the date.