It's easy to villainize mom but everyone else so far as been imperfect. Would confronting and talking things out have been better? Yes but enabling her to chat and date strange men would be infinitely worse. I still think Kousuke is way worse. A half decent person he should at least reassure the mother that he's not a child groomer or a serial killer. He's her childhood friend's father, taking her to the aquarium when she's clearly having troubles at home and school is pretty innocent.
Also I hope Kii-chan will keep Mii-chan safe from hairdye.