IF they are cassettes, I'd bet most likely hum or hiss. Hiss is easier with the noiseprint feature. Tell your buddy that if he can get a decent sized chunk, say maybe 5 or 10 seconds, of the background hiss recorded too (like from right before or after the music) that it would be easier for you to remove it.
The real key to making the NR features work the way you need them to, is to just save the original and then toy with a copy of it until you get the results that appeal to you the best. Use the HELP feature OFTEN on each NR plugin to teach your self what each feature does and keep good notes.
If you let us know exactly what kind of noise it is then we could probably help you out a little bit more. Or better yet, host a small clip...