Ok, first off, that guy is one BIG jerk. BREAK UP WITH HIM. Second of all, you shouldn't get mad at your friend for going out with him because she's probably clueless, so just calmly tell her the truth. If she believes you, great! You guys can both break up with that two-timing jerk. If she doesn't..well, time to get a new friend. If your friend doesn't have trust in you, then she's not a really good friend for you..