If the MacBook shipped with El Capitan, it probably won't boot from an older operating system. Macs typically won't boot from an operating system older than the one they shipped with.
Their Mac can go all the way back to Lion if you make a 10.7.5 installer. And everything since then.
Download the OS you need from the purchased tab in the App Store, use something like DiskMakerX to copy it to a flash drive, then wipe with disk utility and reinstall OS X. Not too difficult. Make backups of course.
I'd expand on the process, but the post is vague enough, so I'll just leave it at that for now.
Their Mac can go all the way back to Lion if you make a 10.7.5 installer. And everything since then.
Download the OS you need from the purchased tab in the App Store, use something like DiskMakerX to copy it to a flash drive, then wipe with disk utility and reinstall OS X. Not too difficult. Make backups of course.
I'd expand on the process, but the post is vague enough, so I'll just leave it at that for now.