For myself I have managed with a small Ashdown Acoustic Radiator - very powerful for the size. There are a few on eBay for around £130 right now, and Ashdown happily service them. Avoid anything with damaged speakers, as replacements were no longer available when I last contacted Ashdown.
Some Acoustic Radiators have a DI out, so your guitarist could just turn down the amplifier and use the DI out and stage monitors if he is feeling bold. However, given his lack of on-stage experience, I would not recommend it.
He can also use the Acoustic Radiator at home, to get used to the concept of stage volume, of adjusting the sound of the amplifier, and of identifying and removing feedback.
Add to that a tilt-back amplifier stand to point the sound at his ears, and a chorus pedal to soften the sound a little. Cables in various colours (so that your guitarist can easily set up: "the long orange one from the guitar, into the chorus pedal; the short green one from chorus to the amplifier.")
Maybe a breakfast-bar stool, if he wants to sit down.
A guitar stand for the guitar, because it's important to set it down without laying it on the floor, or for when he swaps out instruments.
As for which guitar to use, let him test them all with the band first. It may be that (for example) his hand-made custom shop twin neck acoustic is less stage-friendly than a cheaper Tanglewood. He will also want to learn that the rich bass and soaring trebles of his guitars when played solo, may not be necessary in a band setting.
Let us know how he gets on!
Robert
Edit. He will require a footpedal tuner, even if he has a tuner in his guitars. That's because the footpedal tuner can act as a mute switch, to ensure that the guitar's sound can be switched off to the PA, not just when tuning but to kill sudden feedback, or to swap out guitars.