The Witcher 3 Achievements – How to Obtain Walked the Path Trophy on New Game+

Did you synchronize your account on the PS3 on which the trophy was won? If you got the trophy on his console, but didn't synchronize that console with PSN, then PSN won't know you won that trophy, so your own console won't show the trophies.

Get back to his console, log back onto your account, and synchronize. If you deleted your account on his console before synchronizing your trophies with PSN then any trophies won on his console are lost.

Since no one answered and I managed to solve the issue myself, I'll post my solution.

I replayed each level from the last Greece level till the end of the game by choosing Replay Level from the menu and picking the hard difficulty. After completing the final boss the Tough Enough trophy unlocked.

What happens, according to to other people's experience is that if during your hard story mode playthrough you replay a level or do a time trial, your auto save lowers its difficulty to Medium, you have no way to know that but by looking at the difficulty level the game offers you when choosing to replay a level after you've loaded your main save (it offers the last played difficulty). Since I knew where my game changed difficulty, I was able to replay only the levels I messed.

I hope my experience helps others with this issue.