>>The reason why she told him she like someone else because she know OP liked her but she didn't feel the same way. It is easier to say that she liked someone else (which is probably the truth) than to say, "I don't like you".

It's possible but I think slightly unlikely. She could be just treating him as a friend, confiding in the op about her feelings for this other guy.

>>OP your friend is being a douche about this. It is unwritten Man Law that one man will not interfere with another man's pursuit of a girl.

What pursuit? There is no pursuit here. Even if the other guy wasn't interested the op would never pursue this girl. He had a chance, he didn't really do anything.