It depends. If you like him then everything's perfect!! If you don't like him, then dont let him know that you know that he likes you. It'll ruin the friendship!
A better question might be "why do you care if an immature guy who isn't a close friend likes you."
know what he likes and get him something like that or buy a movie he likes but does not own
you will know that he likes your friend because he will always bring her up and he will want to be around her.
They will eventually tell you
don't
Be honest, and ask him.
how he lookes at you
ask him
Your friend tells you.
Woman's intuition. Actually, talk to the guy or have your friend guy talk to him for you.
Tell her the truth.
She likes you and that other guy and her boyfriend but does not know who she likes best.