If you have a friend or coworker with a body odor problem, you may want to tell them about it to spare yourself and others the headache of being in close quarters with them. If the odor is caused by a medical issue, the person may already be aware of the problem, but unsure of how to resolve it. On the other hand, if the bad smell is a result of poor hygiene, the offender may have no idea that there is a problem. Approaching the offending person with tact and concern is the best way to avoid offending him or hurting his feelings.
Ask about your friend's health to find out if there are any health issues that might be contributing to the odor. If she does admit to health problems, ask about possible symptoms and what you can do to help. Chances are your friend is already aware that her health problems can affect her body odor, and she may be relieved to hear that you're willing to help.
Mention to your coworker that you've noticed recently that he's been sweating more than usual and ask if anything is wrong. Excess sweating can be caused by stress, heat, hormonal changes and many other factors. Gently suggest to your friend that he needs to change his hygiene habits to address this new issue.
Ask your friend to meet you for a private lunch and delicately address the issue. Be direct, but sensitive. Let them know that you understand that it's might be embarrassing, but stress that you'd want them to tell you if you had a problem you weren't aware of.