It is a very difficult, if not generally impossible mission, to 'convince' anyone who is not ready to hear the 'truth about the truth'. When a person reaches the end of life, is holding on to hope [even if it is all lies and falsehood], what odds are there of actually reaching them? In most cases, about 0% I fear.
So, why attempt to shatter their hope? Now if they should ask, and really want to hear what you have to say - I would lay it before them. Otherwise, I believe it would most often do nothing but harm and cause confusion at a time when that is not necessary.
Only you know the precise mindset and situation. My mother was Jw in name only by the last years of her life. Still, I refrained. There was no point. There was not enough time for careful debate, and I had no good alternative to offer to whatever hope she believed she entertained at that stage.
I think has she been 20 years younger when I found out what I know, I would have 'felt out' the situation to see if she would be receptive. But in what I knew to be her last months of life - no way.
Jeff