The interior finish on most homebuilts is totally upto the builder and
can be finished to as much detail as you are willing to pay for and/or
give up the weight to. If your friend is not into doing the interior
himself, you can probably find a custom upholstery shop who will create
just about any look you want. The panels can be fabricated from light
weight fiberglass or any light weight stiff material then covered. The
overhead and other areas can be painted or covered as well.