Lavender being a recessive gene means that both parents must either be lavender or must be carrying the gene in order for any of their offspring to express it, and a pure Buff Orpington should not be carrying the gene. The bird in question could perhaps be the result of a second-generation cross...
Do you know what breeds or mixes the neighbor has? Or where the neighbor got their birds from? It's hard to guess on breeds in a mix, especially with young birds or birds in molt, because there are a number of combinations that, when bred together, can result in similar offspring. This is likely...