How to tell my mum to back off a bit?

My Dad has been diagnosed with bowel cancer and is having an operation in 10 days time.  My mum answers for him at every opportunity.  Simple things - he wanted some long sleeved t-shirts for after the operation and she said that he has never liked those, so doens't want them; he said that he is tired and has lost a bit of weight, and she will make a huff or sigh.  I know that this is extremely difficult for us all, but I want my Dad to be able to express his feelings as he is the most important person in this and I don't know how to tell my mum to back off a bit.  I want to send her a text as she won't listen to me if I talk to her and just says, this is how she needs to cope by being positive.