In similar situations I have taken 'wildswan's' approach. I find that enables me to be a bit more like damewithaname (who may be correct, but there is only so much work you can do in a lifetime)
I also think that it helps to remind yourself that you are not responsible for her, or her life. It may be helpful, both for your dear friend, and your son & partner if you make S's life a bit easier, but that should be seem as a plus! I find that this thinking enables me to put emotional distance, whilst nodding & smiling.
Another thing that I find helps is to think to myself 'am I feeling strong today?'. If I am, and can cope, then I am happy to meet. If I am a bit vulnerable, liable to get pulled down a bit, then I try to cry off. This is actually a bit more difficult for me, as my similar relative lives at a distance, and meetings have to be set up - nonetheless, I will cry bad cold / tummy upset if I need to - it enable me to be positive the next time I have to see her (and of course, she doesn't actually see whether I am sniffling or not!)
Although I do not have the skill to be a writer, I do imagine writing her into a book, or what well-known character she would be. I actually find this a good distancing thing to do when feeling a bit battered by someone!