It’s a good place to start with a horse trained from NH techniques. IF the NH trainer is any good, that is, they will have taught the horse to lead past them and go on a small circle around them. That’s a key piece of the whole training system! You point with your leading hand, the one holding the rope, and use driving energy from your other hand, with either the dead end of your rope, a lunge whip, a flag, whatever. When I started working with my gelding on lunging – and all I wanted him to do was lunge, not spin in little NH circles – I had him on quite a short rope doing this, like his NH trainer would have done. As he got more confident, I could use a longer line and make the circle bigger. Voila! Lunging.
You can easily screw up a horse with these techniques by not training the drive aids, just the running backwards and spinning aids. I remember all the videos and workshops of people like Clinton Anderson and Pat Parelli aggressively snapping ropes at horses and making them hurl themselves into reverse gear. On the chance that the OP’s friend’s horse has been taught some useful skills, just with slightly different aids, the thing I’m talking about might help.
Years ago, during a short stint as a freelance trainer, I was working with a horse who had a lot of confusion about how riding worked. Anyway, horse was cooking with diesel on the long lines. Doing really well at it. Then one day, I went to the yard for our weekly or fortnightly session, and the owner said, “Oh, my friend has done some natural horsemanship clinics and then I let her try some of it with Scarlet.”
“Uh-uh,” I said. This horse, who had been long-lining beautifully a week or two ago, was now spinning and trying to face me and reverse every second. In the long-lines, this is not ideal. Took me a bloody age to fix it. Had to go back to single line lunging. I may have given the owner a bit of a bollocking.
that leave you frustrated and the horse worried.