To reinstall windows in this case I would say the best way is to use the windows 10 media on a cd / usb. That way you know you are getting a completely clean install. Click here for the download link.
I would also say that it would be a good idea to completely clean the hard drive first if there is definitely no important data. that way you won't get any lingering partitions. You can access the command prompt from the installation media I linked earlier, then follow this guide to clean the disk. Once that is complete restart the pc and you can reinstall a completely fresh copy of windows from the usb. This will remove all data completely so make sure they are happy with this being done before you do it!
To get to command prompt from the installation media, click "repair this pc" when the install screen comes up, then "troubleshoot", it may be under "advanced options". You can run diskpart from there.