MarkoBoss302

Randolla
YES,  stuff happens.  Components fail all of the time for no "apparent" reason. It could have been a voltage spike or something else that caused the problem or something just failed.
 

To complete what you said:
The PC is second hand. We think the PC stood without use for 1 or 2 years.
This could be a reason to give this problem?





You seem to be obsessed with WHY it happened.  What you should be concerned with is HOW to fix it.  Sometimes the WHY can help the HOW, but whether or not the PC sitting for a year or two, is not important in this case.  You need to focus your energies on HOW to fix the issue.  The only way to do that is through trial and error.  Your first step should be to remove the components from the case and install them while they sit on a NONCONDUCTIVE surface, such as cardboard.  If the issue remains, try a new  or spare PSU. (always use a different power card than the one with the original PSU)
 
 If the issue is still there with a new PSU and outside of the case, it may be the motherboard.  If the motherboard is still under warranty, then the next step would be to RMA the board.  If it is not under warranty, then I would consider a used board.  Personally, I would not put too much money into this rig.  I would consider getting some used X58 parts and go that route if the motherboard is not under warranty.  But that is just my opinion.  That will all depend on what your friends budget is. 
 
First rule out a short and a bad PSU and then go from there.  My guess is that there is a 80% chance that either a short or a bad PSU is causing the problem.