If you import their song from a copy instead of the original that is ok. It isn't your responsability to make sure they own the original and even if they don't own it you will not be the one in trouble.
Many people, my wife included create custom discs to take to shows (other than our own) because I don't feel like lugging around 550+ CDs so she can pick out which one she wants to sing from. Besides I don't like the way some KJs handle their CDs (and mine).
So Billyo it is ok to use someones copies, but I do reserve the right to refuse but that is usually because the disc is badly scratched or soaked in beer.