What do you do when a friend dose an a act that you totally do not agree with?

Last night i was watching a show and it made me think what would i do. so I tried to think to when a friend of mind may have done aomthing i was not in agreement with. A friend of mine started dating a married man. I feel strongly about this because my father has dated many women while still married to my mother. so when my girl pull this mess it vexed me some bad. I mean i had a hard time interacting with her. I thought she was being stuid to belive that this man loved her like he said he did. I kept telling her well he will never leave his wife. Well guess what he left his wife. so this made me go back to her and say that i may have been wronge because every man is not like my father. Well just a few days ago she tells me that he is not cheating on her with his ex wife. she found out from his five year old daughter. Now that is some Karma.
I just would like to hear what would you do if one of your friebds did somthing you did not agree with how would you respond to that friend?