I am not an expert in Natural Horsemanship or Horse training. I am not a master of equitation or winner of many ribbons. However, I do have one small trick which I have used many times before when a horse is feeling like it wants to be with its buddies/pasture and not with me. This was shared to me by a very experienced riding instructor, and I have seen similar theories in various DVDs, Clinics and books also.
This requires your friends horse to stand quietly still while your friend holds it. It also requires a halter and lead on your mare, and possibly a long crop/horsemanship stick/lunging whip/carrot stick etc. You would probably want to start in a safe contained area with alot of room. A round pen would be too small, an arena or flat pasture would work.
First take your horse over towards its buddy. Work it very hard, yielding, lunging/circling, backing, transitions. No breaks, no rest. Make the horse MOVE for about 3 minutes.
Now turn and lead your horse swiftly as far away from its buddy that you can get. Relax. Release all pressure, look away from your horse and allow your horse to stand totally still. If your horses gives a huge sigh and stands on a slack lead beside you, good. Go to A). If your horse immediately tries to return to its friend, then go to B).
A). After your horse has rested for a minute or so, move your friend and pasture buddy then repeat at least 3 times in different places until your horse understands full well that when they are with their friend they have to work, and when they are with just you they get to rest.
B). Do it again. As many times as you have to until your horse is willing to move away from buddy and rest. Then proceed to A).
Once your horse gets it, take them back to the paddock, turn them out and allow them the rest of the day to process these thoughts. Horses learn well with repeatition, so yu may have to repeat this but it is the fastest way I have tried to help a horse get over herd boundedness. However, there are many people on this forum who will also have good ideas, so listen to everyones opinion and try what will work for you.