How do i get a boy to go out with me and dump his girlfriend but even if his girlfriend is my bff xx?

Well i really like this boy but hes going out with my best friend and i dont wanna hurt her but he told me on the computer that he loves me and his girlfriend but i really love him but i dont wanna fall out wiv my bestfriend just for some boyy what do i do :(? thankyou for future advice i get:')xxxxx