It's not obvious to have a great breath and it's also not obvious to have a mouth odour. A great breath takes a lot of care including brushing properly and having check ups with the dentists.
Have you noticed that people with a stinking breath talk a lot and they like it when they whisper so close to you. Sometimes I feel like they know that they have a really bad smell and they want you to feel it.
Here's the problem, do you say, you have a stinking smell from your mouth or, do you know that your mouth stinks? No, I think that's a little harsh. So how do you say it because you cannot pretend not to be disgusted. At some point, it's gonna get out so how do you want it to sound?
Simple. Find the words and be courageous enough to bring up that topic with the other person in a way that is sensitive to their emotions. If you are not strong enough buy a gift and write a message in it.
It's not hard after all. Don't stay quiet when you can actually help. Just say something.