Sometimes people have morning breath or garlic-breath of some sort. If I'm talking to a friend in a public place and I notice this, what should I do? Should I tell them to brush their teeth in the morning? Give them a mint? What is the politest way to tell them that their breath smells?
When someone has bad breathe I take gum out of my pocket and take a piece for myself, and then offer them a piece. It appears as not rude since they can pretend that I was being polite since I took one for myself, but it solves the problem (at least temporarily).
If you continue to do this with the same person they will probably figure it out, so that is something to be mindful of and decide whether you want them to know that is what you are doing or not.
To close friends
Move closer, and hint that their breath smells without drawing others' attention to it.
To not-so-close friends
Probably stand a little further away, and hope they sense why I did that.
Depends on how friendly we are. Refer the abovementioned.
No limit on what I'd say; but not in public though. No formalities.
Don't mention it. I'd do what I came to do, and move away.
I consider it a sign of my own hard-won maturity that I do not expect everyone to be like me and live by my standards. Bad breath won't kill me (not to mention worse things). I should be really sure that the issue is important before I open my mouth and make it my problem, and possibly make my friend's problem bigger (they might already know and feel bad enough or be depressed)?
Of course, if this is a friend, you might say, "Hey, your breath is pretty strong. Does that matter?", and let them decide. They might be grateful that you had the concern to let them know without parenting them.