My best Friend is seeing a married woman with a small child.?

I am trying to make them see that what there doing is wrong and my best friend knows its wrong but says he cant help it. She is Planning on Moving in with him even though hes still married and the wife left him they are in the very early stages of divorce. She still lives with her husband and small child they own a house together but wants to relinquish rights to the house and her child. I am so taken back by a woman so eager to leave her child move to another state just to be with him. I am trying to help them see and my best friend of 20 years values my opinion , How can I walk away without not forfeiting my friendship?