What should I do? If anything?

A girl, a former friend, who spontaneously decided to dislike me seems to have befriended my best friend wholeheartedly; and is influencing him, we always used to just hang out the two of us, but lately she is showing up more and more, and im feeling more and more pushed out. I know thathe, himself has nothing against me; we've been great friends for years, but as she gets closer to him, i get pushed out; and he, himself cant see it!?