So at my DM and Ddads at the weekend.
As we're leaving Mum says to DS 'Don't forget you must get mummy to listen to you read tonight'.
Me: he reads everyday
Mum: That's not what he said - he said your always too busy.
I said nothing, we were in the hall getting our shoes on and said quietly to DS - you must stop telling lies, you know the consequence so there's no computer tonight.
My mum then started telling me off for ruining the afternoon. 
- DS has been telling silly lies lately and he has to know it's wrong - especially when they hint at neglect!
- I said it quietly and DS accepted he was wrong and admitted telling her I was always too busy doing washing and ironing. (which I do when he's in bed)
- and this is the AIBU bit. She was the one who decided to make a comment in front of family and friends without knowing the truth.
So AIBU to think she didn't need to say anything then and if she thought it was a problem ringing me to discuss it would have been much better?
DS actually came home today having not changed his book and when questioned why it's because the teacher has heard him read information books really well and wants to him him read his fiction book as she thinks he can go onto library level. So my reading a variety of texts at home with him is paying off. and I'm fighting the urge to ring mum and tell her!