Buddy sourness is, at its basic level, insecurity. An insecure horse won't become more secure by removing all other horses from her. Jerking a horse away from the herd will only make her more afraid.
If you want to move her away from this one mare, you could. Put her in another pasture with other horses, and you'll find she'll probably pick out a new "best friend" and you're back to where you started, unfortunately.
Chronic buddy sourness rooted in anxiety and fear will only be healed with time and work. I have a mare who was so attached to her best friend she would rear, call, and tear down fences to get to this other horse. I took her on walks in hand away from her friends. If she called and carried on, I worked her. Ground work involving moving the feet and using the brain. I gave her the opportunity to rest afterwards. If she chose to carry on, we worked more.
Ride with your horse's buddy. If your horse starts carrying on, get off and work her on the ground while your horse's buddy keeps riding away like nothing is happening. Work your horse until she's calm and standing still -- not looking for her friend. Get back on and continue your ride. Repeat as any times as necessary.
If this is too much for you right now, have your mare ride out in front. Trot away from the buddy horse, stop, and wait for the buddy to catch you. It's always easier to leave the buddy horse than for the buddy horse to leave you. Riding in front gives a horse confidence. As does riding alone, if you're comfortable with that.
Ride with many different horses if you can. That helps a lot.
It's a long road to a horse that's comfortable in its own skin! It takes time.