We don't really know what her tantrum really was. Was she apologized and compensated for her meal or OP told her "we don't have anyone with black straight hair" (and while he has such hair, heh)
Maybe it was a hair stuck to someone's clothes and that fell of (or maybe it was OP's hair indeed if even he don't go near the food!). Or that blond person's hair looking darker after being soaked in meal. Or someone with curly hair had sime straight strands or they were less curly after falling off. I would assume any of those before assuming customer intentionally put that hair
Maybe it was a hair stuck to someone's clothes and that fell of (or maybe it was OP's hair indeed if even he don't go near the food!). Or that blond person's hair looking darker after being soaked in meal. Or someone with curly hair had sime straight strands or they were less curly after falling off. I would assume any of those before assuming customer intentionally put that hair