Oh, then I guess you meant that you had the iTunes file of their library. I was thinking of the actual mp3 files of the songs in their library. My mistake- sorry. Unfortunately, iTunes uses distinct IDs for each library, so you can't really import it to your own without using third-party software (which is usually costly). But you can enable sharing from their computer and add it to yours that way. The option for that is in the "sharing" tab in preferences:
Or you can copy their library's direct files onto your passport and use the "add to library" button from there. The default directory for iTunes' library is
C:\Users\*username*\Music\iTunes\iTunes Media
If it's not there, you can...