This could only happen if BOTH parents of the chick were impure for the pea comb gene. If I were this lady, I would re-evaluate my selection of breeding stock, especially if she is going to be selling hatching eggs. In her defense, it may be next to impossible to tell which chickens are impure for the right comb unless a test mating is done, but usually their combs are a bit high, narrow, or misshapen, especially in the males. Another thing to watch for is to make sure they have three rows of tiny points, and we should try to select breeders whose combs are fairly wide at the base.