Does it really need to be unrepairable? How much does your father know about computers and what measures will he take to make sure it is non-functional/ try to repair it before letting you give it away for free?
If it's not much, you could do something simple like take out the CPU power cable to the motherboard. It will just turn on and do nothing.
Or do you really need to something unrepairable? Do you need it to, say, blow up or would you be satisfied if it simply didn't work? You could open up the power supply, clip one of the main leads and put everything back together. Your friend could even salvage the power supply by re-soldering it. Be careful of the capacitors. leave the PSU unplugged for a while before working on it, and even so, careful no to touch the capacitors just to be sure.
If you need it to explode, you would need to see if any of the capacitors have polarity. If any do, they can be installed backwards and explode, if you have access to a soldering kit and can desolder. There's always the chance it could take some components with it though when it dies.