Slightly more awkward
If you plug someone elses iPod into your computer, Im pretty sure you can play from their library through iTunes (and hence through the airport express). However if they have any tracks downloaded from iTunes I presume youd probably need to authorise their account in your copy of iTunes to get those tracks to play. I cant test these presumptions though, so cant say for sure.
If someone has a non-iPod MP3 player that you want to play music from... then youre up a rather smelly creek without a paddle. Short of copying their music from the device to your computer first and adding to your itunes library, I cant think of a way around that one.
If that happened, Id unplug the airport express and plug in their MP3 player :?