He was persuaded.

So you're the Kumada-kun that Sayaka often talks about?

Even though you were nursing my little sister, I was rude to you. I apologize.

Oh, me too... I'm sorry to have come in on my own.

I'm glad you've gotten on such good terms with my little sister... ... but

Juuuust kidding... That was a jo~oke.

Afterwards, lots of nursing was given as he was forced to stay.