There are a couple of ways to correct, or at the very least, salvage the damaged threads.

The best is to see the bike shop and have them chase the threads with the proper die. Make sure the shop is aware of the vintage nature and value of the fork. Or...

You can clean the threads up, individually, with a very small three corner file. This will take an hour, or so, of carefully cleaning/smoothing/forming each damaged thread portion up. Every so often, try your headset nut to see if the threads are improving. Or...

You can take an old, unwanted headset nut, or lock nut, and use it to make your own thread chaser. Cut a single groove, at an angle, into the thread of the lock nut. Just slot to bottom of the thread. This nut, with its slots, can be slowly and carefully threaded onto the damaged steering tube. By going slowly, the slotted nut will actually help to restore the thread.

All three of the above suggestions are not repairs. The threads are not being fixed. They are just being made useable but be forewarned - the threads might not hold well, depending on how much material was originally damaged/lost.

Finally, suggest to the owner to stop mounting their steering stem so high in the steering tube. My guess is that the stem wedge expanded at the threaded section, swelling the diameter slightly in the area that is damaged. Just a guess.

Hope that is a help and I have done this sort of thing many times.