Well i think it depends on the said friend,some friends can be very angry if you tell them in a straightforward manner that their mouth is smelling,meanwhile there are some friends that won’t get angry at all,i think you should just start first by giving them hints,do not tell them directly

This is a challenge. It is rather difficult to do so. We are afraid our friend will be offended, maybe even so he will become angry and break up friendship. But as a friend we have to keep trying to help him. The first way may be in invisible ways, for example by offering a candy fragrance or invite friends to eat foods that can eliminate bad breath.

We can also invite to discuss about body odor. Try to talk slowly about this issue. Point him to respond and discuss with us. It's our chance to awaken him to his bad breath.

The last way is to directly invite to speak. Encourage your friends to speak the way they are. Tell her well what she's going through. Maybe you can say something like this "I apologize, no intention to offend you.As a friend I have to tell you honestly If possible, try to cure your bad breath.I'm sorry, no intention to reproach you". Here are some tips from me, hopefully useful.