Her voice is so mild when she rebukes him, that I doubt she is making an obscene gesture.  I could easily imagine it being a forefinger raised as a non verbal request for him to halt.  It's really impossible to tell.
		
		
	 
Ok, I've had another watch/listen.
He steps right in front of her as if she wasn't there.  She says "Please don't knock me off"  He comes back with "Shut up, you <garble>**"  I reckon her finger goes up then, in the classic American style.    He chases her, knocks her down and abuses her.  
I think it's a matter of opinion who started it.  Personally, I'm on her side.  But if I was going to give the finger to someone I thought was going to knock me down, I'd make sure I had a getaway route available.
** I can guess what the garbled word was.