BEST FRIEND’S SECRET

How do I help my best friend deal with having a long term relationship with another woman, while not wanting to destroy his remarriage to the mother of his children? The long term relationship has been going on for 25 years through his marriage, divorce, girlfriend, their separation and his remarriage to his first wife? They are both married, but seem totally, emotionally connected to each other

(Screen) Name: Sunbunny12B