Was it the temptation that whats it like to kiss someone who likes your best friend, tht got to you or just the temptation of a warm feeling? Either way, it was wrong, and the fact that the guy backed off, means he likes your best friend..before he tells her, you should tell her clearly, so that she does not come to know of it from someone else,atleast not from the guy. So sit with her, and tell her clearly what happened n apologised..she might walk away from you and might not want to talk for sometime..but she is right to be angry you should apologise and leave it at that. Let her come back to you if she wants to, you will atleast know she was your true friend or not.