All of my computer science friends who have moved their parents to Linux (myself included) report that they are happily doing with their computers what they used to (sans downloading and running random malware) with much reduced "tech support".
Hell, when I introduced my mom to ubuntu ten years ago, Linux wasn't even as desktop friendly as it is now.
Not to say that it's the best choice, but it's plenty usable and user friendly.
Mine was using non-free applications too beforehand. You obviously would need to find ways that she can accomplish the same tasks as before without too much trouble.
Having said that, if she wanted a new computer right now, I'd probably recommend an iPad with a keyboard case (assuming that she can do what she wants with it, which I think she can -- most of her computer use nowadays is web browsing, email and the occasional word processing/spreadsheet. I use mine for everything except coding)
At the end of the day, its down to cost analysis: is it cheaper (in terms of time spent, for both of you) to leave things as they are and deal with support issues as they arise; or to retrain your mom to use something which has lower support overhead?
Hell, when I introduced my mom to ubuntu ten years ago, Linux wasn't even as desktop friendly as it is now.
Not to say that it's the best choice, but it's plenty usable and user friendly.