So would you be friends with this guy ...or would you say that if hes not sincere to his wives he cant be sincere to anyone?
Well it depends on how your Dad knows him. If he is bringing in money for your business then stay friends! Lol.
Seriously speaking though, I like to enjoy my life and that involves not interfering in other people's business or judging them. I don't like to break up with anyone or hurt their feelings. If it is not directly affecting me in anyway then why should I worry about it. Its not always fair to judge. But if he asks for advice on this situation then yes I would advise him to be honest with first wife or choose which one he wants.
If your Dad enjoys a good friendship with him then he should carry on being friends.