agchristie - your points are well made, and your argument is sound, in my view.
Speaking personally, on the very rare occasions when I have been in the vicinity of any kind of violence - actual, or brewing, my response has been to be moving in the opposite direction as quickly as possible.
If you are standing next to someone who is assaulting someone, and you are not either intervening, or making obvious moves to exit the area, and you are being filmed, then the possibility of you being seen as involved becomes very strong, and the ability to demy or explain becomes seriously weakened.
As you say, charges may or may not be brought, and the legal process will proceed, but as an unbiased observer, it appears to me that this man was involved in this incident, and willingly so.
But we shall see what happens from here.
As for other posters still trying doggedly to play 'Devil's Advocate' - except when they take arguments against them personally - hopefully that has finally run out of steam.