Someone I know who has windows 7 was put on peppermint but after using it they decided they didn’t like it and wanted to go back to windows. Is it possible to re-install windows on a peppermint computer and how?
It's definitely possible, and it's essentialy the same as installing Windows on a brand new computer - you get a Windows installation USB drive, boot from it (plug it in, then restart the computer), and go through the wizard.
There are plenty of guides to do this (Microsoft's official https://docs.microsoft.com/en-us/windows-hardware/manufacture/desktop/install-windows-from-a-usb-flash-drive).
There's also one for Windows7: https://www.microsoft.com/en-us/software-download/windows7
Just be sure to copy your important personal files from the peppermint, because Windows will most likely do a clean install, wiping the drive clean.