There is no source that sells all spare parts. The only two I know of do not carry any parts for that machine. You (aka your friend) have 3 choices. 1) find nearest trash receptacle and toss in botched machine and learn from lesson...buy new machine 2) Pony up and pay ASUS to fix it. This will come with a warranty on the repair 3) Scour EBAY and buy there with the chance that you get a board someone else cooked or cook it again yourself putting it in.
If I were in that boat, which I would never be, I would be sucking it up and sending it to ASUS to repair. Has to be cheaper than a new machine. If its too close to the cost of a new machine its a no brainer just buy the machine. What you dont realize is you are not just buying a MOBO. The CPU and GPU are soldered on so you are buying those too, not going to be cheap!
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein