P.H.
I even tell my son we cannot plan a playdate the same day he wants one, it has to be in advance.
This mom could be so overwheled and need a shoulder..so maybe see about having Her over for coffee or meeting her somewhere to talk this out..try not to accuse or assume as you may not know what is going on..but do not give up on her and her daughter as they may need you as friends and to make her daughter pay for her moms lateness would be unfair.
Even try to give 'fake' times so she could be there ontime?