If they plan to try you on more than one of their originals, don't be shy about asking them for charts. If they haven't even bothered to write out the chords ("oh, this one is really simple, you'll pick it up in no time"), then invite the guitarist(s) to play through the song while you quickly chart it for yourself on the pad you remembered to take with you to the audition.
You'll probably need to borrow a pencil, mind.