How Do You Tell Someone Off ?

I have friends who have foul-smelling mouths. It's not cos they are unhygienic, in fact they are very clean people. Who bathes the 2-3 times a day, even brushes their teeth (and tongue.. I reckon) twice a day or maybe more. It's because of their irregular eating habits and sometimes not even eating that causes this. The gastric juices eating up their stomach and causing the smell.

Some isn't very bad as it only happens on occasions where they forgot to eat or skipped a meal but at other times, they smells alright. I do have this 1 colleague tho that smells really bad every time because he refuses to eat at all. There is a lot of complaings about it but no one dares to tell him. All I could do is tell him that it's unhealthy for him not to eat but I dare not tell him about what it's making him smell like.

Question is, how do you tell them that they stink without embarrassing them? Cos in the process, you would be embarrassed by feelling embarrassed for them.

Any ideas?