YANBU, that's horrible, unless there has been some massive crisis (clutching at straws) then its a shitty thing to do.
I had similar twice with the same woman - yes, I know I should have cottoned on the first time.
First time she calls and asks to take my two DSs out for the day with hers - they go to a holiday thing together, my Dss are a year apart. meant to come at 11 to take them somewhere nice, after calls and texts by which point I'm almost about to just take them myself, she turns up at half 2 and takes them to local park for an hour as that's all she now has time for...if I'd known in advance that's what the deal was, it would have been fine, but making up a picnic and having 8 and 9 years hanging about for hours waiting was really not on.
2nd time was the following summer, by this point her son and DS2 are more friends than boy and DS1 who is a year older. Anyway, she calls and asks if DS1 is free to go out for the day with her son, I say, "did you mean DS2?" she says "No, I don't have room in the car for them both and son would rather invite DS1". Whilst I don't need my children joined at the hip and they are allowed to have different friends and different trips, could she not conceive for an instant how her scenario of a day out would make a 9 year old boy feel? You can insert my response.....