My guy was SUPER BUDDY SOUR!!! I'll post how I helped him in a bit have to charge phone
The only thing I found that works and takes at least two people. Have someone riding the other horse or any horse that is likes and run them after each other in a small circle trotting as fast as you can get them. After they seem to be getting tired move them in opposite directions and let them rest till they are calm. (it doesn't have to be far just so they are looking at each other and cannot touch at all) then repeat this again but when they go to rest go a little father away. You might have to do this more than a few times and most likely more than a few rides. If you are working your horse and they are any other horses around just pick some where secluded to rest. Soon they realize when they are around other horses they have to work harder. You might want to try doing this with more than one horse to yours does not think it is just that horse that makes them work. Hope it helps!
(I know this worked with my barn sour horse who has a little but of buddy sour in him) A way you could try would be by riding away from the "buddy" then coming back to them, then ride away from them again and come back, you repeat this but each time you get ferther and ferther away and come back... (Like I said I'm too sure how well this could work with buddy sour horses but you could give it a try)
I would take the horse away from his buddies for a short time, and show him that he always gets to come back to his herd, that they won't be gone when he comes back. It seems like it's kind of a thing that tends to get better over time.
Yeah. There's no actual fixing it. You just deal with it as best as you can if can't permanently separate them
I've never heard of someone actually fixing a buddy sour horse. I've just heard of having to keep the horse separate from his buddies and living a pretty much isolated life. I've never personally dealt with one.