To counterbalance the iPadists – if she’s familiar with the way Windows works, stick with a similar/the same system. My dad is OK with his old iMac, pretty much ignores the iPad, because it’s all different.
Mint XFCE, Ubuntu, Win7/8[modified, obvs] – anything much the same.
You might be able to setup a Mint XFCE install in an afternoon, and assuming the hardware is OK, it’s likely to run very smoothly. If you’re up to modifying partitions with gparted* you might be able to shrink the Windows partition and keep the original Windows install as well. After install, you just choose the OS you want to load between the POST screeen and the “Starting Windows” screen. With grub-customizer you can change the default option in the menu as well, or add a longer timeout to it [it will autoboot one of the OS’s after a few seconds by default].
Libre Office seems to be OK at exchanging documents with MS Office.
*pretty easy, actually