It is the Etiquette of Sweating: AKA Sweatequette.
Here is everything you need to know.
STEP ONE: Relax a bit
We all sweat. It happens. It's pretty natural. I mean, some people sweat more than others, but don't panic. It isn't the end of the world if you end up with some sweat on you.
STEP TWO: Don't Be a dick
If you see someone is clearly sweating, there is no need to point it out. Unless they are your very best friend and they are in a situation in which something can be done about it, there is no reason to point it out. What is someone with back sweat marks going to do while they are out at the park? Almost nothing other than stress about it. Instead, how about you buy them a nice popsicle and cool them down a bit huh? Be cool, man.
STEP THREE: Treat Others as you Wish to Be Treated
If you know you smell bad, don't run up and hug others. You wouldn't want that happening to you. Or if you are in a public situation like a bus, don't force someone's face into your smelly armpit. Think of how sad you would be if that were you? Be conscious of your smell and sweat levels to be courteous to others.