How do Ты tell someone that their breathe stinks without being rude?

"I'm bored. Let's drink mouthwash "

Depends if that person close to Ты just be honest and direct.

Offer them a mint или chewing gum :D If they don`t get the hint, insist that they would need it

Just be rude, fucking do it

always carry a pack of gum and offer them one.
или mints. ^^;;
i really can't think of a nicer, subtle way of doing so ^^;;

Ask 'em "what have Ты been eating today?" или "did Ты forget to brush your teeth today?"

start with "I'm sorry but"

First tell "Ooh my breath stinks like shit!" Then practice oral hygiene knowingly and drink a fucking glass of water and eat some damn chewing gum.
Then drop by a mint mouthwash and they'll get that they were being a salty smelly af.

"DEUUEAUGH"