I cleaned a pocket watch for a friend and ran it for a few weeks before returning it to make sure it was running good. I gave it to my friend only to have him tell me that it stopped a day or so later.
He dropped it off to me today to let me check it out and when I looked at it the balance wheel was all spastic! It was just shaking a little left and right real fast. Well I suspected it had somehow got magnetized, so I demagnetized it. Within seconds, it was back to running great.
It turns out that my friend had been traveling all last week and gone through a lot of airport metal detectors. I guess that was how the watch got magnetized. So maybe someone can explain how the metal detector would screw up a watch and how to get around it (without leaving the watch home).
It does sound like your friend's pocket watch's balance wheel got magnetized somehow. There are different ways to detect metal and most of them do not use strong magnetic fields. But, apparently the watch was exposed to a strong magnetic field that magnetized its balance wheel and the surrounding steel parts. Watches which have brass balance wheels are a lot less prone to this problem.
So, although you did not give the details of the movement, I would assume that the watch had a solid gold case and a steel movement with a steel balance wheel. Gold will not shield against magnetic fields.
The only way I can think of to prevent this problem in the future is for your friend to carry the watch in some sort of steel case when he is traveling. The outer steel case will act so as to prevent any magnetic fields from entering it and the watch it contains.
Another solution would be to replace the balance wheel with a brass one, but that can be a major watchmaking job.