I have been on the opposite spectrum, i.e I had sex with an ex of my good friend. I had sex with my ex girlfriend s best friend. How mad would you be if your ex-girlfriend slept with your best friend? My friend's boyfriend was almost suicidal after she had sex with his best friend. He had cheated on her so they broke up and she had sex with his best friend. The boyfriend was most likely sad because his own best friend betrayed him, if he cheated on his girlfriend he most likely.