You can't always be the flavor of the month. People have different people in their life at different times because that is whats needed by them at the time. People do drift apart even when they don't have a falling out. Sometimes this changes the physical dynamics of a friendship. It may just mean that this new friend is giving your best friend the support or whatever is needed at the time. We fulfill roles in life without even realizing that's what we are doing. If you are willing to share your best friend then the problem won't seen as bad. Focus on things other than what your best friend is doing. & keep the friendship open & I'm sure the closeness will return. Jealousy is not a great personality trait so try not to be upset or hurt as people need more than important friend to turn to in life. Maybe you can create a closer friendship with one of your other friends to help fill the gap in the meantime. I had a close friend of nearly 40 years who said that our lives had gone in separate directions so that was why she didn't always return my calls so I respected that & didn't keep her up the top of my friendship list as she was busy with other things that were more important to her, & I concentrated on other friends who had time for our friendship. I actually appreciated her honesty. It allowed me to give where my friendship was needed more & that was elsewhere.
Nothing. If you try to interfere, your friend is going to think your just being rude and running off all their other friends. Just let it play out, you wont lose them as a friend just cause someone else is involved, they just have another friend too now. Share, your friend is not a possession, its a privilege.