Thank you all. She does have some health problems and she had to leave France, where she had lived very happily for many years, because of the daft inheritance rules. She has two sons and one has mental health problems which means she does not want him to inherit a lump sum on her death. She has had to go back to England, (fortunately, she had kept a house there, she is not short of money) so that she can set up a trust fund to give this son a monthly allowance. She bought another house in France and spends as much time there a she is allowed as a non-resident. I don't think it is her circumstances that depress her, though, I think it is just her personality. You can tell by looking at us how different we are! I am fair, still quite chunky in spite of losing two stones, and usually smiling. She is very dark, very petite and very intense. I don't think her son's problems are as great as my daughter's - his life is not at risk and she is on good terms with both her sons.
She is immensely kind and I would hate to hurt her feelings, but I have a far more limited budget and I don't want to waste a holiday on cheering her up - I don't think it would work , anyway. She cannot walk very far and wants to visit many museums- I like to pootle around a town and I don't want to visit more than one museum in a day. I suppose I could drive her down and suggest we split up and meet for lunch and dinner.
I don't think she has many friends and I do feel sorry for her - I wonder if the robust approach would work? I mean just telling her that I don't want to keep talking about our problems with our children (I am all talked-out anyway) and I want a happy holiday. It would only be for a few days as she has to get back to England on a fixed date.
Now I feel that I am being mean and I should grit my teeth and take her with me. After all, I have had over 50 years of taking my sister on holiday (at my expense) and putting up with her moaning! I finally snapped with her and she has been better ever since but I am not on such familiar terms with my friend.