Please note on the following:
There has been a huge change in the differentiation between anything older than Windows 8. This applies for Windows 7, Windows Vista, Windows XP, and anything before.
Those three older versions of Windows all have a similar format of finding programs/the layout in general.
Windows 8 changed everything, the layout, where programs are located, how to find them, literally a revamp of the entire system.
Windows 10 is of similarity, but its still a huge change if you're converting from an old OS.
Since she's an oldie, I imagine she would be very confused by the new layout of the system.
I don't recommend you upgrade her PC.
ALSO, the reason her PC is slow is because of her PC being old, it has nothing to do with the OS (for the most part.)
There may be some optimizations with memory usage with Windows10 that her versions of Windows doesn't have, which could potentially make the PC easier to use, but I don't see any purpose other than that. It wont provide significant improvement.
All in all, the only thing you're going to accomplish is this:
-You're going to give her a more confusing Operating Software
-You will notice little improvement with the speed of her PC.