And why someone has to lose more than 900 $ by selling your friend the 5btc for 20k?
dummy you dont lose $900 by selling, you sell for less than the other seller because you are a miner wanting to liquidate bitcoin for cash - which you then buy more bitcoin with for less than you sold, same day!
lots of people do this on local bitcoins - how and why the hell would anyone sell bitcoin ever in the first place?
worthless comment
You dont have to sound insultive as its a legitimate question. You also dont expect someone to make that kind of volume of trade without questions in other to clear the curiosity. Since you are in no capacity to make the trade available to him, I will suggest two options
1. Its either you buy in the US with cash because I dont know how easy that will be, then transfer the bitcoin to him or
2. Since he is on localbitcoin, then I dont see any issue as there are always buyers and sellers on that platform which in can peruse all you can do is to recommend any one you have for him or guide him to be more careful.