I am sorry to say there isn't much you can do about this situation. It isn't that unusual that some women prefer men (or women) that are wild, mysterious and your friend probably has it their mind they can change that person (the tamer.) Not true! You know it, but your friend is going to have to go through the painful process of learning this the hard way and hopefully learning a lot from the experience.
Just be there to pick him/her up off the ground after they realize the person they are in love with wasn't the best choice.
You're one great friend!
Marcy