If I had to guess, I'd say the threaded insert in the middle of the starnut is not grabbing the star part thats grabbing the inside of the fork. I've seen that happen before.

Oh, undoubtedly, so once he can get sufficient leverage on the bolt, it will either jam the insert against the star so it won't turn with the bolt, or the bolt will simply rip the insert out of the star. Either way, the starnut is toast. Those star nuts are not meant to be tightened very much, just enough to set the headset adjustment. Sometimes people mistakenly overtighten them after the stem bolts are tightened, thinking this is necessary to secure the fork and they get damaged like this. Also, if the star nut is secured in the steerer tube by threading the cap bolt into it and hitting it with a hammer to drive it in, it can be damaged. Best way is to use a proper setting tool.

I've seen a lot of people kill star nuts by trying to tighten their headset with the stem clamped down too. It can be a pain to get out but it can be done.

Special ed classes, passed with a D. Explains a lot doesn't it?