Do I tell my friend her husband cheated

[Jaelle]

My friend has been married to her husband for 23 years and they are the parents of 5 children, ranging in age from 5 years to 20 years. They have always been happy and together all the time. A year ago things changed. One of their children passed away unexpectedly from suicide. Their whole world fell apart when they discovered the reasons why and sadly, they blamed each other. They both dealt with their grief in different ways and started to drift apart.

A few weeks ago I came to visit and found her husband in a compromising situation with another friend. Her husband talked to me about it, and explained that they have been seeing eachother for a few months and gave the reasons. He told me he felt guilt for his daughters death and it started as them (the other woman) sharing the experience of losing a child and it "just happened." While I understand his reasons, I do not think it is right that he cheated.

He says he will tell my friend of his infedelity but does not know how or when would be the best time. I think he should tell her now.

She should know the truth, however much it will hurt her. Should I tell her? How do I tell her? Or do I wait for him to get the guts to tell her?

Either way, this will hurt my friend and damage her already damaged mental health.