I think the idea of synchronising is to update your mp3 player with the most popular (i.e. frequently played) files from your computer, so it seems unlikely to me that files would have been copied the other way around, from your mp3 player to his computer... but I never use it so I'm not 100% sure. Try checking your mp3 player on mp3/jpegs etc that weren't on it before.
edit:
http://windows.microsoft.com/en-HK/w...sked-questions
says:
What is sync?
In the Player, sync is the process by which digital media files are copied from your Player library to a portable device, after which you can play the files on the device.
So probably it only works one way, from computer to portable device.