The friend's PC would need to have iTunes installed on it, for the shuffle to be recognized properly.
I assume this worked before, using the same Mac, since you have songs on the shuffle. If you have not since this problem occurred, try shutting down (powering off) the MacBook. You may also want to try disconnecting any other USB devices currently connected. Do this to reset the shuffle
Start up the MacBook, run iTunes, and connect shuffle to a direct USB port.