My best friend ended our friendship because her gf didn't like us being friends. WHat should I do?

Don't worry its not your mistake. I know that is wrong but you know one thing that he is the only one who had lost his best friend. Trust me if you think he was your best friend he might not lived you for her gf.
Such guys loss both his best friend and his gf and i am sure he will realize that someday..
Its my personal experience...So no need to feel bad about it.