"I just wanted to let you know that you're just like me. We occasionally have bad breath! Here's what I've found that helps." I'd then give them mints, gum, or mouthwash. Then I'd add, "The only reason I'm telling you this is that someone once told me the same thing and I realised how much it helped me."
I would ask my friend if she wanted a mint or a piece of chewing gum. I may even ask her if she had eaten any garlic last night, even if I know that she didn't. If she declines then I would say I think it would best that you eat a mint or a piece of gum anyway. It is quite a sensitive subject to broach. Honesty and subtlety is the answer.
There's no way around it, if someone's breath reeks tell them outright, especially if it's friends or family. It may be better coming from you than a stranger.
I would tell someone they had a smelly breath by offering them a minty gum and asking them if they forgot to do something this morning.
I would say, "You know that the food we eat sometimes affects our breath. I also sometimes have that problem and I think you also have the same problem but you can sort it out."
I would offer the person a gum every time we are in company and then explain the benefits of it. I would say that this particular gum is the only one that gives me the confidence of not having to worry about bad breath.