Be fair - she feels sorry and is not a violent person at all.
To be honest if I am having an argument with someone it is not that often in underwear especially when I am not intending to have any closer contacts.
Now let us imaging you are in a heavy argument with her. In psycology they suggest in case of an argument to touch each other as it will reduce your agressivity. Now for sure they knew about that technic and to enforce it she grapped his balls while he touched her ..... (this is the point we don't know).
Well similar to most recommendations this one did not work. A safe distance is sometimes healthier even if the arguments are more emotional.