katt he kissed you back did he not?, he is the one to blame, if appointment of blame is what you want, he knew what this could do, to you and your friendship with your roommate, yet he proceeded anyway, dont blame yourself over this, was a genuine mistake on your part,

he knew what he was doing, its called playing the odds, if he had said no, what are you doing?, then you would have made a terrible mistake, as it stands you are blameless in this, your friend will forgive you,

explain to your friend if it had not been you whom had kissed him, it could have been some other woman whom your friend doesnt know about, at least this way she, and you both know what kind of man he really is.