My best friend, "Joe", dated my wife's best friend, "Lizzy", for a little over a year and they broke up. I know Lizzy pretty well too, and consider her a good friend. When she and Joe broke up, my wife and Lizzy were kinda pissed that I would still go hang out with Joe. I told them both to grow up, and that I'm not going to stop hanging out with my one of my best friends since 8th grade. Lizzy was mad, but she got over it (especially when she got married a couple years down the road). I'm still friends with both parties.
Your friend is a douche if he stops being your friend over a woman. Especially if said woman isn't even on his nookie list, so it just means he's completely spineless when it comes to standing up to his own wife.