Hi there.
My mum is in the same position. (Well sort of!) She was diagnosed with breast cancer 13 years ago. Dementia has only set in over the last couple of years. Thankfully not too bad at the moment, but getting steadily worse. The cancer was under control until a couple of years back. She wasn't getting check-ups by then. She was told to just go to her GP if there was any change. In Jan, when she had a fall and went to hospital, they found a growth under her breast about the size of an apple; the cancer had broken through the skin, and poor old mum hadn't told anyone. She forgets that she has the cancer, but is adamant that she does not want to go through chemo / radiotherapy etc. Luckily her consultant is lovely. He can see the trauma just visiting him is putting her through, so has referred her back to her GP. She has the wound dressed every couple of days, as it sometimes bleeds. In one of her clearer moments, she said that at her age (88) she'd rather have a few good months, than a year of trailing backwards and forwards to the hospital. It is heart breaking to hear that, but I really can understand where she is coming from with all this. Thankfully, at the moment, so can her GP and Consultant.
I think you have to stand your ground a bit in this situation. As I say luckily, mum's consultant seems ok with the situation, whether that changes if things go down hill or not I can't be sure. I did have one conversation with a different consultant, who when I said I thought the upheaval of the treatment would be too much for her to understand and take in, said we'll she'll have to have it!!! (I don't think so mate!)
If she doesn't understand she has cancer, I'd leave it at that. Sometimes my mum asks what the growth is. It was so distressing for her to hear that she had cancer that I now tell a little lie, and say its a just a growth.
Best Wishes.