I personally initially thought Faure (as did at least a couple of other people) --but then I read all the great other suggestions below, and now I don't know. And more importantly, I don't know whether you know! Maybe this is one of those situations in which the journey iS the destination... but if you did determine for sure what she was playing, I'd love to know what it was. Whether you found out definitively or not, I'm glad I clicked on this post and I bet the other 2.7K people who clicked on it feel the same way. Thanks for posting it: your Mom did that cello proud. Signed, a former cellist.
OOP: Thank you for the kind comment. As yet there has been nothing close to what she is playing. I have tried to listen to everything suggested and can't find a match. There are vaguely reminiscent moments but a good match would be complete and not just three or four notes. I have loved the conversation and the journey and I love having some options for matching the audio. I have a number of reproductions from various cellists I just haven't posted them all. The one I chose to post was the most thoughtful and beautiful with the accompaniment, also the best matched with timing in the film. The combined posts have been viewed over a million times now and its amazing to have been able to share the film of my mum with so many, thank you everyone for the experience!
[Editor's note: link to the famous Fauré elegy if interested!]
Can we also talk about the way they chose to manipulate the perception of the data by their choice of states