theory 1: when an eevee breeds, (Lays an egg at daycare) if it was female it turns to Ninfia if it was male it turns into another thing.
theory 2 : trading either holding prism scale or holding something else, or trading for a male eevee, when this happens the person who sent male gets ninfia, and the person who sends male gets something else.
here is why i do not think it is friendship: Eevee evolves through happiness by day or Night, if another evolution comes it would be a roulette too many things to consider.
it also looks normal type. it better not be a dragon.