Perhaps try Web of Trust and McAfee SiteAdvisor.
I can't confirm this because I'm on a Mac, and he's running PC so nothing happens on my end.
Or you are oblivious of a trojan running on your Mac. Unfortunately most of the people I know who run Mac don't install antiviruses, under the false impression that they are immune to all malware. This is far from the case and viruses are abundant despite this false sense of security upheld by advertising hype, misinformation and plain ignorance. I was once a junior system admin at a Macintosh-only office, the amount of malware I discovered that had collected on the network was unbelievable.
Play it safe. Practice safe hex.