Is it the rule that winner of the duel gets to write about it and can write whatever details he wants about other character? It isnt question of changing the outcome, but describing other character's moves, reactions, etc.
To directly answer this question: No. You cannot roleplay what another person's character does.
However, if the game states that you seriously wounded him, you can say
where you wounded him, provided this does not permanently maim or cripple him (unless you have the player's permission to do so!). So you can say you stabbed him twice in the side and once in the chest, but you can't say you cut off his arm unless he agrees to let you.
