I would like to add a little sequence in my mod duel where two former friends and talk there. Can I help one in the script? Thank you. Then there should then proceed normally in the game.

Sounds like you want a cutscene sequence triggered by one combatant reaching a certain hit points (say 5% of their total hit points) during the duel. Shouldn't be too difficult for modders experienced in scripting for the game, it would be a dlg script trigger at 5% HP with a cutscene tag.

I'm not very experienced with this (I've only done it in NWN2) and am caught up in a mod myself, but I'm sure someone can help you out.