Thanks for writing. Without seeing it, there's no way for me to say for sure. But whiteflies can leave honeydew excretions as they feed. A black fungus called sooty mold often grows on the honeydew. If this is what's causing it, your friend should be able to scrub sooty mold, and honeydew, off the leaves with a soft, wet cloth.
---Justin, Senior Garden Editor, BHG.com