What would you do re: Craigslist "missed connections"?

My friend just told me she was reading craigslist’s “missed connections” and there was a note from a guy trying to find a beautiful girl whom he met while walking his dog. My friend was surprised as she read the posting because she realized the woman in question is her brother’s girlfriend (the description was unmistakable). The man who posted it obviously doesn’t know this woman has a boyfriend, he was just smitten when he chatted with her while walking dogs in the neighborhood. My friend and I are debating – should she mention this to her brother’s girlfriend? Or keep it to herself?