What is wrong with people can someone tell me that???  

well there's obviously an emotional pull there and it's hard and very often people go against their better judgement. I don't think you have to forgive them, I think though your entitled to make sure that your fathers well being comes first. If that means hiring a private eye or whatever. I wouldn't normally say that but this woman cheats on your dad and then refuses to stop seeing this guy? She sounds like a real beach to be honest. You have to understand your fathers position but make sure he's not being manipulated by her. You know it's one thing for a guy to forgive his fiance but it's another to accept a situation where it could happen again..Goodluck

To me sounds liek he need to grow up and face what he know to be true and either dump her or stop talkign behhind her back. If it was me she woudl eb down the road as asoon as i found out she was cheating on me.