My friend is sleeping with his crush, yet he is due to be married soon. What should I do?

So my male friend got engaged on new years and his wedding is a few months from now. But his been sleeping with his crush and they have been messing around since last year. He will not let the girl go even after his engagement... How should i advise him?
My friend is sleeping with his crush, yet he is due to be married soon. What should I do?
Post Opinion