Has she tried a methodical elimination diet? Did the tests look for other types of reactions besides the IgE antibody mediated immediate response? As that all that is usually tested for and is not implicated in acne formation. You need tests that look for delayed responses involving other inflammatory responses.
I think the neck acne is likely a food intolerance. But it might not be responsible for all her acne. Mine was a response to citrus, something I nearly always ate. It's common to be intolerant to peanuts which also a age the digestive tract.
Things to try that you didn't mention include vitamin d and mint tea.
As far as I know, yes the tests were just immediate response. I'll ask my friend who is a doctor if she can get the other ones fairly soon. I take vitamin D but she doesn't. She does work outside all day but uses natural sunblock like crazy.
We have soothing tea with mint in it, but not mint tea. We like Chinese tea like tai guan yin or cliff tea.
I would remove peanut butter from diet see if that changes anything I know it did for me. Controlling insulin levels and stress. I have been drinking cactus with aloe gel for regulating the blood. Cinnamon, grounded chia, papaya, bluberries. These things have helped me reduce my severe acne. Cactus is underrated, I think that's the key thing that has helped me the most.
I'll try the peanut butter thing. She'll be hesitant as other butters are very expensive in Canada. May have to wait until out next trip to Whitefish. She eats viatamese cinnamon all the time and papaya. Where do you get cactus? Does it come in a juice?