Tell your friend because you feel that it would be unhealthy for him or her to continue in a relationship where such a selfish behavior is present. Sit them both down so that you can act as a mediator when they talk it over. Tell your friend as soon as possible. You can't imagine keeping that knowledge a secret. Confront the cheater! You might also take action by slashing the cheater's tires or egging his or her house - all in the name of protecting your friend, of course. Keep it to yourself. Statistics prove that your friend will find out eventually. ▶ Next Question ◀