Once people loose their teeth, in my experience, they stop having bad breath - unless they have a digestive problem. So probably Methusalah with his nice false teeth wouldnt have bad breath. Oh wait....they didnt have falsies back then, your loving family had to chew up your buffalow meat for you first, then give it to you nice snd soft😊!
Diet can make a difference too, drinking milk can make breath smell shortly after.
You are doing your friend a favour. He probably needs his teeth looking at, or he is not cleaning them properly. Too much plaque which attracts bacteria is the main cause of bad breath isnt it?
Edit: just be honest and say only a real friend will tell you things that are difficult or that you dont want to hear. Anyone can tell fairy stories to please people and be popular...we know that more than anyone!