The nightmarish thought is that it was a traditional style, where there is a shaped washer under a nut, that is supposed to engage on a thread extending out of that hole. The middle type in the below:
If that is the case, there should be something sticking out of that hole, which there is not, and your friend may have some bigger guitar related issues.
I seriously hope its not that scenario.
FrankenLab:
Turning tone wood into expensive sawdust since 2016!