I personaly think that you should try to have a one to one chat about it, or at least try. Since you guys are best friends or at least were, she should be willing to at least text. I think that you should tell her, that "Hey, I don't want this to get in between us its okay if you date him. or if he dates me can we still be cool." but if she keeps being mean about it, I would say take a break with each other, and then come back together. because me and one of my best friends did that, and were going stronger than ever. About the guy, don't let him get in the way. Long lasting friends are more precious than anything. And I know that from expierence. Having a best friend means that you always you have someone to go back to, and will love you always. A true bestfriend is there no matter what, and if this girl is dumping you for a boy she isnt worth it . :)
Praying for you!