Did... did he just teleport there? Because there's no way he'd make it in time otherwise.
Btw that's crazy that she's worried about the fact that they're growing apart when she still hasn't solved the moving problem.
My best guess is that he was already running towards her (and opening the window) when the panel switched from him at the window to her stumbling, so he was already on the balcony when she slipped which barely allowed him to make it in the nick of time.
But yeah, an extra panel to sell the motion would have been nice. But I guess the author just wanted that suspense shot at the end of the page.