Do I tell?

OMG!!! I was out last night with a couple I have known for over 18 yrs, we have been solid friends and through a lot together.
While we were eating the wife let something slip to me, while the guys were at the bar. She told me that there 16 yr old daughter wasn’t his!!! WTF? Now i love these guys and the husband is not only friends with me, but my older brother too, has been for years. I am feeling like she has passed her burden onto me and leaving it for me to carry. He has another child with this woman (a younger son) makes sense now as the 2 look nothing a like. Anyway, should i say something to him. I feel this is just so wrong on every level. Can’t understand how she could do this to him, lie for all these years not just to her husband but her daughter too. This is going to smash their family apart but its not fair to keep something like this from either of them. The wife was very matter of fact about the whole thing, even after my reaction, she dismissed it as a big deal, when i asked if he knew she just said to me looking at him “had to do something to hang onto him”. I can’t look at her now, even speak. Am dealing with that, but should I say something to the husband, more for the daughter’s sake, she is not a child anymore???? Help??