Ethical dilemma

[Sloe Gin]

It may have been in her current post as she knew what it was and wanted to keep it with her post at all times as comfort that her husband was still writing to her.
I know you said her late husband but in her confused mind her husband may still be alive and that letter could be her link to him.

I was thinking the same as Guy.

With regards to why it was being used as a drinks mat, I think the family member would have just opened the drawer and used anything. Clearly she was aware that it would be a good idea to protect the table surface, but she would have used anything. 

But can you be 100% sure of this?  Isn't it just possible that she had a moment of lucidity and recognised the letter/envelope for what it was, and took it out of the drawer, only to forget it again.

In the circumstances I would just put it back in the drawer where it came from.  There will come a time when decisions have to be made about what to do with her ephemera.  In the meantime, if you want to share  what you read with other family members I think that would be fine.

[susieroe]

Could you put it (complete) into another envelope and write on that: ' To the executors. Do not destroy this letter. Please contact [your name]'. I do wonder though, as you have Power of Attorney, that you will be an Executor of her Will, or have to manage Probate? If that is a possibility, you will be able to decide what happens to it then.

[Zaphod99]

There have been some good comments here. The letter still serves as a coaster. She doesn't like change. In that respect she is very observant. If we try to leave her a reminder note for anything, it disappears over night. She doesn't recognise new clothes and house items as her own, but last week she identified a lady in a photo from 70 years ago, who had always been a mystery. The mind, even when damaged I'd amazing.

Zaph