How do you tell an employee that they have body odor?

Tell them nicely by saying something like

"It's quite warm at the moment and lots of us seem to be sweating rather a lot, maybe we should all take more precautions? So don't feel offended because I'm asking everyone this"

If you tell them that its not personal it should be okay, besides its better to tell them then walk through the office feeling guilty you have said nothing.

Hope i helped xD