First of all… calm down.
Going downhill is really difficult!
How is the rider’s position? Is she leaning forward trying look at his feet as he’s going down hill? Because that’s really going to throw him off balance. The rider’s spine needs to be aligned with the trees on the hill - straight up.
GIVE THE HORSE HIS HEAD AND LET HIM CHOOSE HIS PACE AND PATH. This is the safest option.
It is possible he is a little arthritic in his hocks… He could be trying to sit back a little and shuffle with his front feet on the uneven surface… HAVE YOU EVER TRIED TO CLIMB DOWN A HILL WITH AN UNEVEN CRUMBLY SURFACE - YOU DON’T PICK YOUR FEET UP ONE AFTER THE OTHER EITHER - YOU KIND OF SHUFFLE TO KEEP YOUR BALANCE.
Your friend should consider giving her horse a break and getting off and leading him down the hills. There is no shame in this - Endurance riders do this ALL THE TIME to save their horses.
I really do think people tend to over-react with these kind of issues that can be solved by looking at how the person is riding in the first place. - The horse is sound at liberty and according to the OP moves freely and well. It really does sound like he has some confidence issues with going down hills. Get off and lead him.