When you're accessing the explorer/iTunes folders, there is a file parsing that read the tags. You should try to spot which ones and sent some of them to see.
If he used to use traktor as you once mentioned I´d say that simply fucked the tracks on analysis.....10€ it´s not a tag-problem but tractor-fukked-up m4a-files.
I'll ask him. As far as I know he only has the delivered Traktor LE installed (not even sure about the installation) and I am not sure if he analysed the tracks. I will investigate! Does Traktor write into the tracks or does it manipulate the tracks by default?