Billing while outside of hospital stay

My mother recently went in for knee replacement in September 2017, She was officially admitted on Monday 11th September, however they took in over night the Sunday the 10th. This day was not the official day of hospitalisation. However Lancet went an took bloods on the Sunday and billed my mother on her to savings which were already depleted. on speaking to them they tell me this is none of their business and I must take it up with the Dr and we are required to pay. this is not the first time I have had such and encounter with Lancet. their services is beyond bad to a point where , I do not want their services at any cost