I don't see why he wouldn't help her.
Kazuki's a sullen shut-in whose loss to Sayaka in middle school turned him away from the extroverted, proactive person he used to be. But we still see multiple examples of him still helping her in some way or other and being there for her when it matters.
It'd also be good for him character-wise, because not stepping in here and now just sets him back, proving he's an emotional child who will continue running away. So if he's meant to be growing, then he can't turn away here.
I don't know if it'd be against the relay rules though, but I don't think that would matter given the 'cool' factor at play that is him helping out the super popular girl when she stumbled. And they're on the same team, so.
I guess another option is someone else stepping in, potentially even introducing a Triangle plot angle. But for some reason chapter 11 feels too early for anything like that.