The Josephson C700s is one of my favorite mics for anything acoustic, we've used them on pipe organs, baroque flutes, ouds, and whole jazz combos playing live in a room. Also works as a bloated LDC (a very nice sounding one) if you just use one of the capsule's outputs. My only worry would be the balance between the guitar and voice. If the artist has that under control, then you'd have a great time with the gig with this mic and a minimum of fuss. If not, then, well, it'll be an uphill battle.