One other reason more studies would be useful, is that for the person I know, this happened after the first dose.
That means that at the moment there's just no reasonable way to decide whether it's safe to get a second dose or not. If there were studies that would show that almost nobody has a lasting head pain post vaccination, then one could conclude it's probably just a super weird coincidence, and take a risk with a second dose, to at least get the full protection against covid-19.
Without data the most prudent thing is to do no further harm to oneself, and live with increased risk of infection and all the restrictions and increased costs that are put on unvaccinated.
[Posted in case it provides a data point for your friend to discuss with a local doctor, not to debate this doctor's affiliation, credentials or motivation. Would love to see more presentations of physical evidence from autopsies and lab tests, to complement statistical studies.]
That means that at the moment there's just no reasonable way to decide whether it's safe to get a second dose or not. If there were studies that would show that almost nobody has a lasting head pain post vaccination, then one could conclude it's probably just a super weird coincidence, and take a risk with a second dose, to at least get the full protection against covid-19.
Without data the most prudent thing is to do no further harm to oneself, and live with increased risk of infection and all the restrictions and increased costs that are put on unvaccinated.