What is a good surface material for testbenching a computer?

Yes, it's fine. Basically, anything not conductive or susceptible to static electricity is fine.

Friend of mine didn't trust any surface and just hung all the components by strings from the ceiling.

Wooden is great for its nonconductive properties. Covering it with a hard rubber mat can be very helpful to minimize dings and bums and to prevent sliding. You should be able to find either mid-quality cabinet covers or even the foam matting used to adhere carpets to hard flooring. Apply a light coat of mild glue to the wood and just lay out the mat.

A flat mat is best (as opposed to a mesh) as a mesh can "eat" screws and small bits unless the mesh is fine enough.

I have a computer that I took out of it's case because I wanted to fit in a bigger graphics card than could fit in the slimline case. I didn't want it to just lay around on my table and collect static, so I put it in a old printer paper box that was laying around:

Bringing a whole new meaning to "cardboard box"