I know the knife is pointed at her. That doesn't change the situation much.
Assuming the Nanjbu apartment store is 13 stories tall, (based on the image, it's much more considering the windows and how it towers over them) and assuming that it's not larger as layers on apartment stores tend to be, and let's say each floor is 15 feet because that's slightly higher than average. (Malls tend to have bigger floors, but this is just to prove a point)
This means they cleared ~200 feet (195 to be precise, but there's also the fence) in 60 seconds, or about 3 feet 3 inches in a second. (3 feet and 4 inches with the fence) Considering how this was UPHILL at a 90 DEGREE ANGLE, they were moving pretty damn fast, and moving across a flat surface could reduce this substantially, as was just demonstrated by them running from the other side of town to the store.
Being as generous as possible, they should be able to get to him before he'd even be able to process it, and even if he BEGAN to stab her, she shouldn't be too deeply cut by the time he's over there and started wrestling with the knife. These guys are moving insanely quickly, and should just be in his face before he knows it. Also consider that, to their knowledge, it's a two versus one fight, so if both of them charge and one goes for the girl and the other goes for the knife, the dude will have to give up one.
So yeah, they're idiots.
@Trov