The decisive stroke really was mentioning her own mother's death; Yvette has already seen that the girl doesn't know the trends and is very tomboyish. And the reason for that is very clear once it is clear she has no mother to have taught her such things - at that point it's no longer Dylan's fault for not learning and squandering her natural beauty, but it's because she never had the opportunity.
She'll going to become surrogate mom in an instant.