So, fair use actually does have a limitation where something that would otherwise be considered a fair use may not be considered as such if you know (or should have known) that the material was not legally acquired. (For example, certain copyrighted material may be fair use to use in classroom settings for educational purposes, but that doesn't mean that the teacher can pirate it.)
OK, so then the next question is: would YOUR use of your FRIEND'S copy of a song count as a legal acquisition?
The answer here is generally: no. Unlike with something like a book (where you are free to loan the book once you've bought it due to the first sale doctrine), when you buy a song digitally, you aren't actually buying the song, but buying a limited license to listen to that song. That license does *not* include loaning the song to others (because there's no way to enforce that when you give the song to someone else that you no longer have access to the song. Contrast that with a book...if your friend has the book, then you don't have it.)
So, your copy of the song would be piracy, and thus would not be eligible for fair use.
The final question is whether what you are describing would be fair use at all anyway. And I don't think that using part of a song for an intro would count as fair use anyway.
