If she's been dieting, weight loss can result in some bad breath as well. Of course, that doesn't help you tell her. Tough one, tbh. This may be one of those honesty situations where you just have to bite the bullet. Chances are, she may already know from someone less nice.
You can playfully or lightheartedly tell someone their breath is bad. Like someone said earlier, if you're doing it smiling it comes off not mean spirited. If he or she's reasonable they won't get upset and should take measures to fix it.
Me, I'm self conscious about my own breath. I'll show up to appointments already chewing gum, sucking on a peppermint, or with something to drink. Dry mouth breath can be bad, too, and I hate going too long in the day without having something to drink and having that 'gunk' feeling on my tongue and mouth. After a quick kiss I'll even ask if my breath is bad if I think there's any possibly that it is (i.e. it tastes bad to me).
I think more people need to be more 'breath' conscious.
Thai food, while delicious, gave a lady I saw breath that would knock the flies off a port-a-pottie truck. Our usual time to visit was right after she'd have lunch.
Changed our play-time and also offerred to buy her lunch/dinner afterwards. Problem solved.
Just play it off by grabbing yourself some gum , and to avoid being rude, offer her some. She may have halitosis, however, and this may just cover the smell and not fix the underlying problem.
Altoids are awesome. Buy some altoids. Give her one. Tell her you heard that if a woman pops an altoid in her mouth then gives a bj, that it creates a minty, tingly sensation on your weenis. That'll get her breath smellin' good. LOL