Disgusting after sales for size exchange

I purchased my mother in-law vans, uk6. She fit it on and walk from the room to the lounge and back to the room again to take it off as it was not fitting. The one shoe is clearly bigger than the other. When my mother in-law went to the store to exchange the size, with the slip, they denied her and said she wore the shoes. They also lacked customer service with their interaction with her. I will never purchase anything from this store again. The after sales is disgusting!