Approaching the topic of bad breath with sensitivity is important. You might consider saying something like:
"Hey, I really value our closeness, and I thought it would be best if we could share anything with each other. I've noticed a slight issue with breath at times, and I wanted to mention it because I care about you. It might just be something you ate or a temporary thing, but I thought it was worth mentioning. How do you feel about it?"
Being gentle, non-judgmental, and expressing concern for their feelings can make the conversation more constructive and less uncomfortable.