Sure fine, maybe he was trying to stop her from pursuing. I even thought of that while watching the video. This isnt like dropping someone on a set of normal train tracks though, you can easily roll off those and we well in the clear, these you have to hope you weren't injured in the fall in a way that would keep you from standing and that you are also tall enough to climb out.
Also can't they charge him with all three? Assault, Robbery and attempted murder? Doesn't he still get time with the first two sticking without question? Whatever the out come it needs to be more time then just Assault and Robbery because it wasn't as simple as striking someone and running off with their stuff, she was thrown into a seriously dangerous and potentially fatal situation.