Since you have placed the word "best " in front of friend, it is your duty to tell her everything no matter how new the relationship is between that guy and your best friend. Do not hide anything from her. Its better to tell her everything clearly so that she doesn't suffer later. And moreover from your question it seems that the guy in a way is cheating on your best friend, by showing interest on you. So just warn her. She deserves a man who will be loyal and trustworthy. Redefine your best friend tag with an honest saying since you have observed everything without much hesitation. It will good for you both.