acoustic guitar players can be unrealistic in their want for amplifiers and preamp's, IMHO. They want their guitar to sound like Kottke's "6- and 12-String", Dylan's Its alright Ma, and Steve Miller's Take the Money and Run with the turn of the knob, but they have transducers or piezos, a general purpose PA, and outdoor patio instead of a ribbon mic and killer mic pre's behind it in a studio.
add voice to it? good luck.
a single amp that can do all of those well? maybe have him buy a purpose built solid state amp for accoustic/voice, and you can build him a 6watt champ that he can mic when he wants to Joe Walsh it.
If he has a sound hole pole piece magnetic pickup, you might be ok with an electric guitar amp (depends on his desired sound)
If all he'll ever want out of it is a solo act in a coffee house, maybe a 2x6v6 10W Princeton with 2 channels?
If you have a amp you've built, have him play accoustic through one channel and a mic through the other.... see if he is still interested...
Also, should you move forward with this, you'll probably want to design the voice channel with a specific microphone type in mind (the kind he uses). mic output levels vary (2mV to 1V!), an SM58 has an XLR balanced output @ 150ohm and a level of -51.5 dBV (0.002v rms), depending on the mic... think about a input transformer, and a dedicated channel that you can amplify the vocals up to loud level without all kinds noise and preamp-distortion. and hope it doesn't squeal like a banshee with the slightest feedback.
If he still isn't dissuaded and you aren't either... then lets do this..!