I'm going to tell a guy at a Magic the Gathering event that he smelled bad and needed to shower next time
Have you told people they smelled bad before?
Well this time I felt like I was going to vomit.
It can be uncomfortable to be around people with strong body odor, but it's pretty rude to tell a stranger that they stink. You really don't know what caused him to smell bad, and pointing it out to him in the middle of an event will make him self conscious. Just move to a different part of the room if you find the smell intolerable.
But he need to be taught that a person has to care about them self.
I agree, it is better for them if they smell good, but you should be gentle in the way you tell them. Don't call them out in front of everyone, but bring it up to them in private, and be polite. Telling someone their body odor makes you want to vomit is unnecessarily cruel.