S.H.
It is not tit for tat.
You do not have to make-up to her... for the previous grump she was and circumstances.
A simple apology to her, should suffice.
She is snippy, to expect you to then, extend yourself beyond your means and have her kid with you and tell her kid that he can stay longer... for the afternoon.
The Host of the play-date... sets the rules for the time spent there. Not the guests. Nor does the host "have to " accommodate the extenuating circumstances of the kid's parent. Much less, at the last minute.
It was the MOM, who has to "explain" to HER kid that he had to leave early. NOT you. For her to put the "blame" on you and to explain to HER kid.... was passive aggressive and rude.
Plus, SHE turned up at your house, late. Her fault. Not yours.
A play-date, is for as long as YOU want, or are able to.
It was wrong for her to assume that her kid was going to stay there all day. Geez. That is really rude.
When you have a play-date... ALWAYS ALWAYS state the end-time. Always. And for me personally, I always tell the parent 1/2 hour prior to when I actually want the play-date to end.... because, delays always occurs. But that is just my strategy.
AND, ALWAYS get their cell-phone number so that you can call them, if need be... even if to remind the Mom that pick-up time is soon. That is what I do.
My play-dates...average about 3 hours. Sometimes even 4 hours. And I a solidly explain the end-time, clearly.
All the best,
Susan