I'm not sure of many circumstances I'd feel compelled to relate such information, unless the person is determined to be close enough to me when vocalizing for it to be an issue. In these situations, I usually back up a step. Personal space or bad breath, let them wonder.
As an aside, bad breath, in my estimation, isn't caused by lack of brushing or "germs" that just happen to be there and for which humans need all kinds of aids to cover up (mouthwash, breath mints etc). It's caused by what one eats, drinks or smokes. Much like one can smell alcohol on a person's breath immediately after they have had a few, one can also smell the partially rotting corpses that weren't fully digested from a heavy meat eater, the remnants of which can linger in the digestive system for quite a while.
I became aware of death breath (of others) soon after going vegan. Their meat heavy diets weren't quite as noticeable before.