How polite are you...?

include it in your conversation. and tell the friend you care about them.

mixing with some funny comments or something

tell them like it is but dont tell them in front of others

That's difficult, because you know that no matter how gently you tell them, they will be hurt. Just make sure it's done in private, they don't need to be humiliated too.

Say for instance, I would say it's sad to be born with bad smell, people don't understand them, but there's something they can do, etc I use the word they and them and not "you" Just make up a story. Always say you are sorry for them and what they're doing is not what they really want and what's happening to them is just temporary.

wow that would be a hard one. its hard hurting someone especially a friend.

ok so i'll go this way- "Look i really hate to break this to you but as a friend ive got to tell u this rather than u hear it from a stranger. the thing is that your mouth...um..it well...stinks. sorry but thats the truth. dont feel bad, ok?"

Are you asking for advice, or do you just want to know how I tell em?

Well the best way is to choose your words carefully, and make suggestions, not give orders. Like say maybe a shower will make you feel better

But me, I just say "you stink, take a shower" No point in beatin around the bush jus get it over with, they'll get over it later on.

blunt as trauma
but they know I care, so they take it in the best way
I have no friends who would not do like wise for me

Dude you smell. Might want to go wash your azz. He is my friend after all.