Win or draw...and when?

So my brother played this game recently and I thought it'd be a good one to post. Is the final position a draw? Was it a black win at move 41? If so, at what point did black blow the win?

The final position is certainly a draw provided white doesn't trade his Knight for the bishop : he waits and after Black plays ...e4 he trades and sacs his Knight on the last pawn.

As for the position after Black's 41st, it's very difficult to give a definite assessement, but I would think Black should have some winning chances by bringing his king to the queenside and trying to push his d-pawn ? Hard to tell "cold" though - maybe white has some blockade...

It looks like Black's last try for a win was 54...e4 !? where 55.fxe4 (?) Kxe4 gives White to think. Looks like a draw though.

After 43...Kg7 aiming to reach f5 as fast as possible and then zuzwang White into giving up the f4 pawn it looks winning to me. But I might be wrong.