If you or your friend feel threatened enough to feel deadly force may be necessary, you should probably have already informed the police. Let us pretend that the gentlemen does indeed show up at the event, does indeed incite an argument or physical confrontation, and someone is forced to draw and shoot. Your comments and questions here clearly suggest you or your friend are in fear of your life from another individual. Weather there has been a clear and direct threat, consulting a police officer will at least provide you with proof you attempted to have the police handle or deescalate the situation before it got out of hand, as you were clearly fearful for your life.
In the cover your ass realm of the world, the above suggestion at least covers your ass and creates a paper trail if you are forced by the threatening individual to draw and possibly fire a weapon.
My personal thoughts, anyone who clearly knows I am armed and prepared to defend myself with a firearm who attacks me physically is either crazy or wishes death by citizen.