You can promise the dying anything to make them happier. Anything. Lie your ass off if it is in the direction of their happiness.

Do not take it as a true commitment. This is not an intrinsic perspective. I spent many hours with my father dying and being honest. I would never lie at that moment. I realized that was a mistake. There were occasional moments where he had realizations that he didn't need.

M is a geriatric nurse. She interacts with dying people every day of her career. Some have just got the news and others are actively on the way out and she takes care of all them as they go further and further away into their minds.

And she lies all day. She is incredible at it. She will spin stories for these people of how their family were just there and they'll be there tomorrow. And because these people have lost their short-term memory, they are calm and happy as opposed to crying. Where are my loved ones! Never tell a husband that his wife is dead or vice versa. They go through grief every time. The list goes on and on. Do not cause emotional pain, lying is perfectly okay here.

So yes, she is perfectly coherent now. Of course you won't lie to her. But you might sometime in the future, and it's okay.