My friend slept with her ex boyfriend's friend now exbo is current bo and he cannot move on! ?

My friend's ex boyfriend and my best friend just rekindled a relationship after 4 years. He just recently found out that 4 years ago while they were broke up that his best friend and his ex "current" girlfriend slept together. During a drunken evening the best friend of the boyfriend told him to not get back together with her because he had slept with her. When my friend was confronted she did confess that they did sleep together but it was because the best friend had lured her into it by telling her about lots of her boyfriends indiscretions. Cindy and Ex-bo are now back together and he appears to love her but he keeps bringing up the fact that she slept with his best friend. What should she do?