1) Disable AHCI driver in register while trying to boot in AHCI mode.
He get's half a point for doing IDE compatible, and full score if he repairs reg file and switches to AHCI 😀
2) Put old BIOS so that MB will get "upset" it's got unrecognised CPU 😀
(BIOS update from BIOS/DOS utility training).
3) Wrong initial display (forcing off integrated GPU and plugging monitor to MB)
4) Wrong date + bad CMOS battery...
5) Don't secure one of the RAM latches (make it visually "not right"), PC may still boot/work, but you can check this way if he is thorough (he must secure latch before finishing).
6) No themalpaste under heatsink/bad mounted heatsink (throttling/lackluster performance in bechmarks).
7) Always minimum multiplier on CPU (be it from Windows Power Plan settings or hard locking CPU multi in BIOS).
8 ) Not installed drivers for device(s) in device maneger (MEI driver or Smart Connect on newer boards).
8a) For hardcore level :
Uninstall ATK0110 driver (if PC needs it), and force him to find what device is not installed 😀
There are SOOO many options/ways to do this...
Other thoughts :
X) Don't know how old PC you want to use is, but IDE cable select or Slave/Master can be good training.
Y) Making bootable pendrive and installing Windows from it ?
Last thing :
Show him at the beginnig how it works/scores when properly configured.
That way he will know what to look for/what he might have missed.
If he's novice that is (he will know he missed something and will feel he needs mre training), if he claims to be a "veteran"/advanced user - show him on other PC [same spec], but AFTER he's done repairing his PC.
EDIT (Master Level) :
Low performance after some point in LOAD duration test.
Basicly : Throttling on CPU that requires lower than stock Vcore or better VRM cooling to not occur in benchmarks/games.
Best hardware for the job : AMD AM3+ Bulldozer/Vishera with cheap/mid range motherboard 😉
If he goes for lower frequency only, ask why and how Vcore and Frequency are related to TDP/Heat/Temps/VRM's.