Be friends with the boy and his girlfriend and once they breakup u can have him

I am not sure if you're asking a question or just making a general statement out of the blue.

The answer to how to get a boy that has a girlfriend, is this. If you have morals, and common decency, you leave him alone. He has a girlfriend. If you have no morals or decency then you do what you suggested.

That sounds like a pretty sleazy idea to me.
If a boy has a girlfriend, you leave him alone.