I know the maths , I'm a qualified sparky.My speciality is fault finding and trouble shooting cowboy installs.
I had already worked out everything you said as soon as I looked at the machine. I also worked out from him saying he bought it from a mate for 200$ what sort of 'customer' he was;
'no Enthusiast' is the answer.
I concluded the psu being HP o.e.m. (probs out last the board with stock equipment) will likely handle being pushed to the edge occasionally.(The figures quoted are playing METRO , remember,it wont be playing that 24 /7)
And the psu will likely fade slowly, by which time having pulled it apart once to put the gpu in, when it starts to crash at max power the poster will realise he cant get out of buying a psu and putting it in, and he can now do it himself.
Some people learn the hard way. Praps he only looking short term. When he uses the new card he will realise the machine isnt crap and worth extra $30.
You cant teach em everything in one go, you just do the best you can.
You did well to try tho, but in my xp you are shouting without an audience.