You probably know this anyway...
But you're unlikely to get a shock from anything beyond the mains connector on the back of the PSU, the largest voltage differential inside a PC case is between the +12v and -12v lines and you would have to reach in and lick this to get anywhere near a shock. Most of the internals run at 5v or 3.3v.

You can quite happily work on 50V with bare hands, we do at work all the time and god knows our employers are not into ignoring even the most tedious bits of H&S. 120V does give a proper tingle up the arm if your pliers aren't insulated but even then there's no H&S mandated protection or special tools for working on it.
(fridgefreezer inserted a topical reference into his sig on, Fri 20 Feb 2009, 0:34, 1 reply)

Exactly
'Real possibility of electric shock to him or his customer'.. my bell end!

'As H&S adviser I am very concerned' so I will not even bother asking the name of the person involved, let alone say "Oi! Don't you think you should unplug it first?". Better to just quietly email someone about procedural matters...
(Veni Vidi Lurki had attitude constipation on, Fri 20 Feb 2009, 0:55, closed)