Did the mistake change anything about the outcome of the fight? No, but telling the players that would be metagaming
I would just challenge the rules lawyer to pistols at dawn and point out that most states still recognize the right to duel in controlled circumstances
I feel like pistol dueling is banned in the UK, but we can probably scrounge up some big knives and call it a sword fight
Hmmm, I can't seem to find this so-called "rule of cool" in the official rulebook
Rule of cool is just a bandaid for systems that aren't fit for the group's purposes. That's why it rarely shows up outside of D&D.
"oh hey you're right. thanks, we'll do better next combat. anyway, moving on"
Narrator: immunity to the mental trait would, in fact, be forgotten again. Repeatedly.
rules lawyer
You get one warning before you are asked to leave.
I run PF2 so rules lawyers are generally very useful... As long as they correct the rules at the time, rather than 2 and a half hours later.
Systems that use more than two shapes of dice fail basic usability and will not appear at any of my tables.