That is what the proof of his account was either not secured (2FA Enabled but not was Activated) and his Email also was either in control of others or 2FA was not enabled.
We doing trade of just 100$ we use to activate 2FA and till now everything is well and every time we are been notified how could he invest 1500$ and just leave it its just like you have a Diamond Necklace you keep in your room, you are staying in your leaving room so windows of your room are open so anyone could hookup and take it out so latter in night you cant say that I was in my house but still my Necklace lost.
When you have 1500$ worth necklace in you room do you leave you window open? of course not. In such case you would be blamed not that thief.