I slept with my friend's roommate and now they wont speak to me anymore.
Why did it upset them that you slept with their room mate?
I would not speak to a friend who fucks my roommate either.
Well, you can sleep with whomever you want. But it might have been better to talk through this issue before anything happened. So you should talk with your friend and be honest with them.
Why? How would that help me?
Your friend might have thought it was rude you did what you did. Maybe an apology and talk with them about the incident. They could have had feelings for them that you didn't know about.