As a general rule of thumb, I usually recommend Linux Mint to beginners. The installation and update processes are easy and intuitive, and there is a ton of software available, as well as good support if you know how to do web searches properly. The main trick is to try and remember that a paradigm shift needs to happen here. Linux is not Windows. It doesn’t work like Windows, and it has different aims and priorities. She will also need to be prepared to learn a bit and be slightly more hands-on with her computing. The learning curve with Mint is comparatively gentle, but it does exist.

This is all very broad and general, but I hope it helps. Good luck to the both of you. I hope you are satisfied with whatever you decide on.