I knew this old couple was just being protective watching out for their precious employee as if she was their own daughter. Miyoko was being nosy out of concern. In the end she trusts Kaoruko's eye for boys, because she saw that Rintaro is a true good boy, one I would never count as a delinquent. Some say good guys finish last, but not in this manga. The good boys are winning here. I love to see it.
Kaoruko got invited to a date with her boyfriend at the end of a busy day, and the knowledge she'd get a treat from him: one of her favorite cakes. She might think she's getting spoiled, but I do believe she deserves it for being such a hard worker. She doesn't have much free time, so it only feels right that Rintaro makes the precious time she spends with him truly worth it.