You don't mention how they read these books, is it a Kindle e-ink device, a Fire Tablet, or some other way? I ask because the shitification has started, they have a new file format they deliver to devices that are capable of KFX. That's anything made after the 1st Paperwhite e-ink Kindle. The new KFX file is not crackable with current NoDRM/DeDRM tools that can be found on GitHub.
As for FOSS, Koreader can be found on github and is installable on most Kobo devices as well as Remarkable devices if going with e-ink, and can be installed on android devices through F-Droid or manual install. Always read the install instructions to make sure you understand the install procedure on e-ink devices.