There was a longtime ago a Eomon Migration with something similar and in the end the Reputation to loose by not sharing made him do the right thing.
Maybe Messi does not need that but I doubt that the community would forget it.
OK it was 12 Years ago but I still think proudly about the commumity and the power it had and the looter gained my respect:
Worst Team HoF ever:
https://www.planetcalypsoforum.com/forums/showthread.php?75412-Worst-Team-HoF-ever
As far as I can read in the original thread and this one you failed to :
Present him the rules, ask him if he agrees with the rules and acknowledge them, you invited him without getting his response on that.The mob was not shared loot so you basically invited him in your house.He couldn't do anything about it outside the team.So instead of taking responsibility you blame him , you had the ability to limit his power into your group.If you were the team leader and you invited an unknown and he stole your item without accepting your rules then you were the one that should have paid the other group members for your sloppy decision.Mindark set at your disposal mechanisms for you to control the situation but you choose to let everything random and hope that everyone will do as you think not as you act.
If you ask if its your fault for everything that did happen in that period.
If the guy did accept your rules and understand that he had to share the item then in this case he deserve to lose all his credibility and you can do whatever you want with his reputation.
There are 2 different scenarios as far as I can see them (your case and this one) however you just want to point out someone "stole" you something and you wanted to remind him about losing your reputation, we get it.
Reading few more comments I see this :
"I specifically asked the team leader of the rules, dmg stack share or spilt ?
And the answer was: "dmg stack share rule".
As i see the team leader decides."
I'd trust Lightstar words & actions above 99% of the forum users (unless hard proofs are being added to the table ) but that's just my personal opinion.