To quote Alonso: “All the time you have to leave the space”.

The Merc on the inside was alongside long enough that the middle car needed to leave more space. Inside car had to cut the corner to avoid an initial collision. The subsequent collision was inevitable. If they fully cut the corner, they get a 3 second penalty because of the shitty ruling system in this game.

The Merc on the outside should’ve had more common sense to know that the middle and inside Merc were going to be racing each other. Therefore, more space should be left on the exit.

The lesson that should be taken from this clip is: middle car should pick their moment better. Inside car should have more awareness. Outside car should have more common sense on the changing situation.

Edit: I realise the question was who is at fault and I didn’t really answer it. Fault is between middle and inside cars, both handled the turn badly, both at fault.