Headaches from only certain vintages of wine

[Kcwhippet]

My guess it's indeed mainly the histamines. The amount depends on the particular vintage's growing conditions. Some years there is more histamine than in others. Also, consider the tyramines, found along with the histamines in red grape skins. The tyramines are known to trigger migraines in some people. Both may be what are doing in your friend depending on the vintage.