Since there are not enough details to discuss a specific case here, the general option you are talking about is:
Strafgesetzbuch (StGB)
§ 199 Wechselseitig begangene Beleidigungen
Wenn eine Beleidigung auf der Stelle erwidert wird, so kann der Richter beide Beleidiger oder einen derselben für straffrei erklären.
Translated:
Criminal Code (StGB)
§ 199 Mutually committed insults
If an insult is reciprocated on the spot, the judge may declare both insulters or one of them exempt from punishment.
Note the may. It is up to the judge. This law is about situations where two people meet in person (or phone or video chat) and one starts insulting and the other answers immediately. Then a judge may say "you know what, you yelled insults at each other for 20 minutes over the fence, lets call it even and not bother a court with it".
There is no official weighting or scale to be used in this, since it is a "heat of the moment" thing. If you had time to wonder what the legally appropriate counter-insult would be, you are probably no longer "on the spot". The legally appropriate defense is ignoring their insults and suing them.
If they called someone member of the HJ then I'm pretty sure a judge would let it pass when they called them "cunt" in return on the spot. If they waited two days to write in a paper about it, then it is illegal, no matter what the original insult was.