Shepi, do they really have actual theory on 2...Nf6 3 cxd5 Nxd5?
Yes, the theory is simply that you should play 4. Nf3 instead of e4?! (which allows Nf6 Nc3 e5 with some counterplay).
There is also some small theory on how to meet 4...Bf5 (preventing e4).
Frequently if black castles e5 is correct, while if white can castle quickly d5 will kill an uncastled black.
Here is a game with e5, it is viable but I believe it should be delayed as shown:
I was going to mention the exaple you shown. But if he misses one move or gets the move order wrong one disaster after another will happen.
Patience is the mental reason he lost. The mental reason. That will improve with time. Then he would be ready for that example.
Plus it should not work on a 1900 ranked player.