If you realized that you fell in love with the wife of his best friend, first try to understand your own feelings. Find out whether they are serious or you are just experiencing a fleeting feeling of passion and mere physical attraction. The only way you will be able to understand how you act in the future.
If your sympathy for his best friend's wife is a banal feeling of passion and lust, it is not necessary to destroy other stable relationships. Imagine that you'd be able to get at desirable girl, you get her the best friend, and after intimacy just lose interest in it. Such an act, you not only hurt the fairer sex, but lose your most loyal friend.
If you understand what true love spouse your best friend, you need to pay attention to the relationship of these young people. When there is harmony and agreement, any of your actions will only cause negative consequences. You can just ruin a good pair and again to lose his faithful companion. Just imagine the idea that when a person truly loves someone, he needs to care not that he was good. A man in love should want to make his loved one happy. Leave any attempt at rapprochement with the girl his friend, may they enjoy each other and enjoy family life.
If you realize that your friend's wife is unhappy in the marriage, you can proceed to active actions. First and foremost you need to talk to my best friend, to apologize to him and honestly tell him about your feelings for your partner. Tell me what you can't nothing to do with them, as has already been trying to forget his wife. Explain that it hurts you to see that your friend does not appreciate such a wonderful girl and you would be able to make her happy. However, be prepared that your friendship will end.
After talking with your friend, you can start to get a girl. Surround her with attention and care. Make her understand that you are much better than her husband and is able to love and appreciate her. Admit his feelings. If you start to tie a romantic relationship, most likely, the woman decides to divorce. Just remember that if you do succeed, you will need to protect his beloved as the Apple of his eye. Otherwise, all your actions will be in vain and you'll just ruin her happy life than cause hatred of the fairer sex towards you.