The mutations within any given generation are never going to be so great as to prevent the individual with them from mating within his population -- otherwise the mutation immediately disappears from the gene pool. If a member of the "daughter species" never mutates enough to become "reproductively isolated" from other members of the "daughter species," how then does it become "reproductively isolated" from the parent species?
The entire population of the daughter species is reproductively isolated from the parent species. It doesn't work with one individual -- you need a breeding population.