How is her short term memory? If she's going to forget whatever you say very quickly anyway, and the truth is going to be upsetting, then IMO there's no,point.
My mother was told by her GP that she had Alzheimer's, but had forgotten by the time she got home maybe 15 mins later. And while she'd accept whatever the GP said (doctor being = to God in her mind) she would angrily deny that there was anything wrong with her if we reminded her - and to be fair, I'm sure she believed it. So we soon stopped trying to remind her, especially when she was going to forget inside 2 minutes anyway.