iPod Nano Skipping Songs

I am wondering if she has synced with a friends computer and is trying to listen to her friends downloaded from iTunes music. I had connected my iPod to a buddies computer and gotten some song that he had grabbed from iTMS. I could listen to his purchased but it would skip right over mine. Upon syncing with my computer my stuff would play and skip his. Just wondering if this might be the problem.

She has to delete the friends songs off the iPod, or her friend has to authorize her iTunes.

not sure how you would tell. I would just start by asking her if she has gotten any music from any friends. If she says yes then you have an idea of what the problem might be. If not then the songs could be encoded badly and would need to be deleted and re-ripped.