Hmm, the Christian in me says that you should ask them kindly if they are aware of their odor problem, and let them know that it may be noticable to others... Noticable odor can be the sign of a unpleasant health condition -- internal or external -- so they may need to seek medical attention, as well.
On the other hand, it could be a sensitive social or cultural issue, too. Are we talking, rank-like-feces, rank-like-huge-sweaty-guy or rank-like-too-many-garlic-burritos?