The guiding principle of martial arts is SELF-DEFENSE. It is not intended as an offensive art; no responsible Black Belt will ever knowinly teach this art to someone who has the intention of using it as a weapon.
Consider, it is far, far easier to kill and maim; than it is to simply beat up. The ‘Beat Up’ is an advanced belt training level, and it typically found in the movies.
The lower belts learn to break arms by hyper-extended the elbow, blinding the opponent (permanently, as in sticking the fingers into the eyes until they hit bone), breaking the knee with a kick placed do cause the knee to buckle and let the opponents weight do the breaking; they are taught to strike the throat to rupture the juglar, the knife strike to rupture the diaphram (ie make them suffocate on their own blood), and to strike the skill at the bump in the back of your head (take out Obdula Oblongata).
Is this what you are telling the instructor to do; teach a student bent on revenge, so that he will kill another?
Having taken years of this; I agree with the teacher. Martial arts are NOT like the Kung Fu flicks where people get slapped about and fall down, then run off threatening to teach them a lesson next time.
Martial arts are designed to Kill, much like a sword. You do not beat someone up with a sword.
Sounds like she's raising 2 boys without a father around. If he was he would have had his son deal with it after the first time.