Ray,

If you had another mic, you could actually simultaneously record both sources. Close mic the vocals with the SM 57 into channel 1, (You'll probably have to dial down the lows due to proximity effect). Hard pan channel 1 left.

If you have another mic, plug into channel 2, for the guitar - hard pan this right.

In RB, do not record as a stereo pair, but record the L/R into two different tracks - potentially using the control room L/R outputs into the inputs on the Audigy.

There will be some bleed but if your friend has a need to play the guitar to give a good vocal performance, this is one way to skin the cat.

Does the guitar have a pickup in it? If so, then you're nearly there - record the pickup instead of the mic'ed guitar.