He Wants Me To Tell Her I Can't Be A Bridesmaid

Side 1 says... ok...i am supposed to be in my friends wedding in less than a month and my boyfriend/son's father told me that i need to tell her that i can not be in her wedding..and if i am, he'll leave me. I have been friends with her for 8 years and have been dating him for almost the same length of time. He hasn't liked her from day one and when i ask him why he doesn't like her he just says because i don't. i don't want to tell her i can't be in it, especially since i have so much invested and i know it would ruin my friendship with her.