I think everyone is missing the main issue here.
Your friend tried it on with her, got shot down, then she went straight to you after. So him feeling rejected to then hear her getting banged by his friend who probably knew about the rejection (to his mind anyway).
You need to explain to him that you didn't know what had happened between them and if you had known what she did you wouldn't have slept with her because you value his feelings over 5 minutes.
Your friend tried it on with her, got shot down, then she went straight to you after. So him feeling rejected to then hear her getting banged by his friend who probably knew about the rejection (to his mind anyway).
You need to explain to him that you didn't know what had happened between them and if you had known what she did you wouldn't have slept with her because you value his feelings over 5 minutes.