how to install mac OS (Sierra) from USB drive, I am out of options
Follow the instructions at What to do before you sell, give away, or trade in your Mac ...there is a link halfway down this page on How to erase a disk for Mac. Yes, at one point you need to put in your girlfriend's or her father's Apple ID, but that's only your permission to install the OS from the App Store (you need an account = Apple ID to do that). The Apple ID you enter at that point does not stay on the system at all, it's only used to get access to the OS installer on the App Store. There are other ways but this is by far the easiest, given your background with Macs. Best regards.