Not sure if it's what you meant, but I had a similar story where I reacted to everything, even the control. The allergist took that to mean "you're really allergic to everything," but I went looking for a second opinion who was able to get both the positive and negative controls to work. Turns out I'm not allergic to everything, the first allergist just didn't recognize benign dermatographia and was a bit too quick to ignore my positive reaction to the negative control. In all, it saved me a good chunk of change and sensitization risks that would have come from having unneeded components in my allergy shots.

I don't know the details of your case or if any of this applies, but if by chance you were in the same boat wrt the negative control it might be worth circling back for a confirmation test at some point.

I'm exaggerating a little, my partner has extensive allergies but not absolute. I jokingly say my partner is allergic to life but in reality it's just life on earth.

I had this test done. Your partner needs antigen shots they really help, ask their allergist for this therapy.

I'm not soliciting for advice, my partner has been seeking medical treatment for allergies for at least 30 years.

well you never mentioned antigen shots, so thought I'd share a n=1 therapeutic that worked for me. Allergies suck, hope your partner finds relief.