Hello
@Angelcat
I'm really sorry to hear about your mum's diagnosis and understand how upsetting it will be for you.
"don't know what to say to her"
Unless your mum wishes to talk about it, I wouldn't`t mention it. When my husband was diagnosed, it was generally accepted he had problems with his memory but Alzheimer`s and dementia were words not used until he asked about it.
Just try to be yourself, a loving and caring daughter, and follow your mum`s needs.