Does she cook? If so, maybe some decent knives, or a nice olive wood chopping board or rolling pin - something that is a timeless classic. Nobody has ever objected to a le creuset casserole dish either (if in doubt I would go for that classic orange colour they have) - or if your budget doesn't run to that, the versions you can get in Sainsbury's are actually just as good and just as attractive in my opinion.
Or a nice stonewear mixing bowl - again, a classic!
Or bakeware? Does she make cakes at all? One of the best gifts I got recently was good quality bundt tin - I had never made a bundt before so it has been fun to learn about them, and they are actually really easy....
Nice photo frames are another possibility?
Of she drinks alcohol, wine glasses?
Or some good quality towels? Maybe it sounds boring, but I think everybody loves a really nice big fluffy towel that will wash well. Your son can hopefully give you a pointer as to what colour she would like.....
Can I just say it's lovely that you are going to this amount of effort to make her feel valued? Really sweet. In her position, I would appreciate it.