My little Bros Karate club.

[Kain]

according to law, if someone is attacking you you can use appropriate force necessary to dehabilitate or retain them.

So if someone calls you fat with a knife to your throat you could easily get away with killing them as most courts would find the situation life threating and therefore you could return the force equally. Besides all practioners of martial arts are required to have a martial arts licence which states that they are legally allowed to use their teachings in defence without any consequences coming of the fact they may know more than the average Joe.
Sueing problem... Solved, kinda...
btw i do Judo and Ju jitsu and have been doing so since i was about 5.

Congrats to your bro mate, tell him to keep it up