then you can just download the microsfot windows 10 download too, it will find the right version for you, download a iso or create a usb stick iwth the installer for you
do backupd of their important files stotred on the hard disk, my docments, desktop files, downloads
ll you can find form her profile
if she has licenses of some programs like office try to find them first with tools like produkey, you need another installer for those and depends on the exact license she has
then format completelly the hard disk with dban or with the windows installation tool delete all the partitions and then let windows to create the partition for you
if windows does well, it should add drivers for everything so you don't have to hunt them
then copy files back
how much ram does that laptop have? 4 gbs at least? if not, consider buying more ram
if the laptop is in windows 10 32 bits, you could downlad the 64 bit version, it is better in the long run