Zack is close enough to get hit with that phantom pain and to be easily overheard when he does, echoing that Ow out loud. Of course, his is more from surprise than pain - he isn't the one punching rocks and stuff as it falls out of the air, after all, and isn't prone to randomly feeling hurt by... nothing?

He's hold his hand (the same one she's shaking out), when he notices her, but doesn't make the connection right away. (Because assuming it's a mutual feeling would be kind of crazy, wouldn't it?)

"Hey! How's it going over there?"