It's the mother of one of DS's friends. Several times she has texted me to ask if DS is free to come and play with her DS. I reply with suitable times then she will say she is unwell or something has come up and could I have her DS instead. She has mentioned that she is undergoing tests for something which they never seem to get to the bottom of. I don't want to not help her out but I can't shake off the feeling that she's at it somewhat and never taking her turn. Latest example was yesterday, she texted and asked if DS was free. This morning she asks if I could take her DS as she was unwell. I replied OK then and would call round for him in half an hour to which she then replied that she was at her Mum's house (her Mum being at work) and could I collect him from there. This meant jumping in the car and driving 3 miles across town instead of walking round the corner.Her DS is a nice wee lad and plays with everyone but when it comes to playdates then he only ever has these with my DS or with one other boy in class. It's not like we're best buddies or anything, we don't even socialise together.
Start saying no. You may feel bad in the beginning, but you have nothing to feel bad about. We had "friends" who'd do this. "Will you pick up my dcs from school, with yours? Helping each other is whta friends do". No, we have arranged childcare, like you should have.
It does sound like she is taking the piss a bit! Don't encourage any further plaudates but if she does invite your ds round to hers again say 'yes that would be perfect timing as I have some studying to do that evening/ have a haircut booked' etc so when she inevitably does cancel at the last minute you can easily say 'oh that's a shame bug I've got this haircut booked, I can take my ds but I canf manage both, sorry' and she will hopefully get the message once you do it once or twice!!