I have never been asked to give a breakdown on how we spend my son's DLA! And yes he has been assessed by social services. DLA is to be spend how you see fit.. whether it's stuff he needs, activities he enjoys, or goes into the household budget. I'd be tempted to say 'gin and cigarettes'.
In adult support, I do believe DLA is taken into consideration (for personal assistance, living support etc) but assuming your son is younger.. no she has no right to ask and no reason to be told.
You can tell her that that he receives DLA and that it is used as seen fit! (or tell her to mind her own bloody business!)
An update on today. I received advice from a support service for parents of children with additional needs. They told me that it is wrong and due to the SEND reforms my son is entitled to non means tested support.