Real-Brisker:
I disagree with you. I think mytake has a very valid complaint against the girls, and would have been right to chew them out.
Chessed is completely optional, but once you agree to do it, and the other person relies on you, it is an obligation. If they had simply refused to do it, or told Mytake they would be an hour and a half late, Mytake could have taken a cab or made other arrangements.
The key is that Mytake acted in reliance on them and foreclosed other options. Asking for what you were promised is not being ungrateful at all.