UNKNOWN
That is not a matter which has been properly explored at any point, during testimony or cross-examination   which of these other persons would have been able to contain this considered attack.   Testimony is that it occurred in a second, a matter of part of a second and in view of the training of police officers we are not looking at who should have reacted.   A person who sees something happen, must act regardless of the fact that four persons might be present.   If all of us took this "let's wait and see what happens" position, then it might have occurred that the attempted blow by Mr Biko might well have hit its object, that the situation might then have escalated beyond control.
	Considered the other way round, your question that the opposite of your question is just as important.   What would have happened if no-one acted?   Would it not then also have led to a situation of chaos!