There is no definitive - it has to be 'reasonable'. You simply cannot legislate for every evantuality - so someone impartial has to look at it and say - Yes, that was reasonable.
From the reporting of this case, most people will say, what the lady did was reasonable. Her friend was being threatened by an implement, so she picked up an implement and threatened back.
This is not comparable at all to the previous case discussed - and I will not bother repeating why he was not being 'reasonable' in that case.