Do you still have contact with your friend? If you brought him here or told him to reply to the questions asked above, you'll increase your chances of recovering them. So, if you
really want that money, do your best and help us understand the whole case in a disgustingly detailed way.
The keys of your money are most likely not compromised by a hacker, because they've remained untouched since 2016. I'm pretty sure that a hacker wouldn't keep them on their known address, but mix them instead. They'd have also spent them all these years.
As NeuroticFish said in the first reply, some money were sent bunch of weeks afterwards to that address. Does your friend remember of making three transactions from 13uz during that period?
I asked him if he did a backup to his wallet and he told me that he only imports the private key to send the coins.
What do you mean by “he only imports the private key”. Has he written the private key(s) anywhere?