How to troubleshoot overheating and high battery drain in an Android Device?
BBS is more confusing than GSAM battery companion. It provides more detail, but GSAM + root can tell you a lot. If you could provide screenshots of full charge, to fully dead we can point you in right direction; if this happens in 7 hours with the screen off and no interaction all the better it should be very trivial to find.

From there, you could use Xposed App Opps module to disable apps with excessive spyware.

I don't really help people (relatives, friends) with this stuff unless they're willing to understand 'why' themselves, easier to just recommend iPhone.

There's almost certainly a rogue app running around doing something. A picture would be worth a thousand words