One of my best mates, Sam, got engaged to his on and off "girlfriend" Katie and are planning to get married next year.

Neither me or his friends approve of his engagement, but Sam insists they are in love and that she's the one, on and on... any time one of us tries to reason with him and point out all the red flags he gets really upset and ignores our advice.

When someone asks Sam about what kind of girl Katie is he says she's a hippy(lol). And everyone agrees once they meet her. She loves going to raves and festivals. She's also a hardcore feminist (facepalm).

More examples of red flags:

-Katie used to date Chad. He is her "ex", alpha Chad sleeps around with groupies and doesn't give too many fucks about Katie.

-Sam is aware of Chad and knows his past and current relationship with Katie. They are basically fuck buddies aka AF/BB.

-Sam is OK with it(wtf) since he's still fucking Katie on the regular too.

-Sam says Chad is going to move away soon.

-THEY GOT ENGAGED OVER THE FUCKING PHONE. Sam was out of town and Katie calls while he is away (probably making sure he isn't getting action from other girls) and they decide over the phone to get married next year.

-Sam just bought a house with his cousin. Yet he is thinking of moving out to live with his new wife eventually...

-Sam insists that they are soul mates and whatever bullshit Katie fed him he's bought into.

This list goes on.

Thing is Sam is a stand up guy. He gets along with everyone and strangers become his friends within minutes. I love Sam like a brother and I would hate to see his inevitable fallout with Katie. If they really do get married he's fucked.

How can I help him? I don't know what to do. He chooses to be okay with everything that shouldn't be. Katie has his balls and manhood tucked away in her little fucking bag full of kitty litter and bullshit.

Please don't say let him learn because this could ruin his life.

Edit:

Tldr; Friend ignoring advice. Marrying the absolute worst girl. How to help him?