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.
It's a close relative of the "I don't feel like looking the relevant rule up, so let's go with the spirit of the law" that shows up in 3.5 and in GURPS.
"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.
No I need my emotional support polyhedrons
As a dice goblin, I support this sentiment.
Tabletop Rpg posts, content, and recruitment posts.
Recruitment posts should contain what system is being played, CW for any adult/serious themes players need to be aware of and whether a game is beginner friendly.
An obvious reminder of no racism, sexism, ableism, homophobia and transphobia.
Emphasis on small independent rpgs like the ones in the TTRPGs for Trans Rights in Texas but not against dnd stuff.