how do you politely tell someone to do something about their bad breath?..

You could try and get them to accept a stick of gum or tictacs if you have some yourself. Any way around talking about the issue would probly be embarrassing for most people.

If I would be in your situation, I would not tell the person directly for fear of offending him/her. What I would do is to try to introduce a product. For example, I would say, "Hey, there's this product-- a mouthwash (for example)--that's really cool! Let's try it together." I just hope you could get to brush your teeth or gurgle together, though. Another is that you try to show some action that you are conscious about your oral hygiene by being conscious about your breath. That person may follow suit.

it's really difficult to tell people they have bad breath... i think the best way is to offer him a gum when you're aroud him