Go with Fedora KDE. it'll be the most polished DE and a good starting off point for transitioning from Windows. Mint is fine but there can be issues. Ubuntu is a headache especially when it comes to updates and upgrades (experienced it on my server this past weekend) and their sneaky "we'll install a snap instead" approach to some things I just don't like.
Fedora is easy and a good middle ground to get them into Linux. Also since you already use Fedora it will be an easy transition for both of you since you can provide first hand support.
someone likely borked IAM.