Agree. Tell them!

Depends who is your person-
Close friends and family : Tell them :) We tell them a lots of things. Of course how you say to them depends on your chemistry with individuals.
Office Colleagues : Its tough. Sending them unknown notes is anti HR :). You can always talk in private asking if they have any issue or problem. Again very very careful.

The best way is to personally tell that person politely in private so that he/she can do something to improve it. There are cases where the person himself is not aware of his unpleasant smell but it bothers all others around him. In such cases we must counsel those person privately to make him aware of his unpleasant smell. There are many types of treatment available nowadays.

This conversation should be done in private. Make sure to show sincerity and empathy. If this is a coworker, involving HR may be needed to ensure no harassment claims are being made.

I agree with most of the comments above. Have a conversation in private and discuss business implications. Delicate manner is required. I would start the conversation if any "personal issues" at home that are affecting them? Not an easy discussion but had to do it once about 20 years ago and was fortunate enough that the person I had the conversation was open and did not realize his/her smell.

This is just one example of a experience someone near me had.... He have usually really bad mouth smell probably because he has Barret syndrome. A coworker ofered him a mint candy and he refused it and the coworker told him: please, take it, it is not for you, it is for me.....
They had confidence so it was funy, but......may not be the best way, but sometime funny things makes serius issues easier to manage.

Thanks, Maria. My dilemma is that she is the one who should be addressing this issue to those who also have obnoxious smell. She holds an administrative position and has no sense of humor. Her subordinates also complain , bring air fresheners, etc.