It's the fault of the person who forgot to repair before entering the instance, but them having to leave because of it makes things more difficult for the remaining players who now have to wait for a replacement, which could take a little bit if it was a healer or a tank who messed up. So far I've only ended up in a dungeon with someone who had to leave due to broken gear twice; one time it was the healer, and the other time it was the tank. So it would have been nice if someone else had been able to offer them a repair.