Sculptor's molding materials.

You could use clay around an armature, he'd have to have a big kiln for a scupture that large, and know what he was doing, as far as thickness/drying etc.
A better material might be cement or concrete. Air-drying, works around an armature, like clay. Google cement or concrete scupting for detailed info.
Depending on what he wants from the finished product, there's papier-mache or sawdust/binding agent mixes. Less permanent, not really suitable for exterior installations, but would be okay for some purposes.

No, clay is terrible on anything that large. You can't get any definition either.

You could use sculpey, it's expensive, but if budget wasn't a problem he could do it. (look it up, it's super common, you can buy a good sized brick for under $20 at any art or craft store if you want to get a feel for it)

If he used sculpey it would be over an armature, most likely the armature would be metal.

For information about polymer clay like Sculpey, try here: www.glassattic.com

The orange cat in my avatar is something I made from Sculpey 3. It's good for small objects, but I wouldn't recommend it for something as large as human-size. I know polymer clay if you have questions, and I have a limited understanding of earth clay. Polymer clay cures at low temperatures compared to earth clay. Polymer clay cures at around 275 degrees Fahrenheit.

Sculpting has two basic strategies: additive or subtractive. Additive is using something like clay where you're adding material to the thing you're building. Subtractive is using something like stone where you're taking away material.

For larger pieces in metal, there's something called a "lost wax" technique. I think it can be used for large pieces; in an art class, I made something hand-held. In this technique, the statue is made from wax, then a mold is formed around the statue. The wax is melted away leaving a hollow space inside for the hot metal to be poured into. The mold is then broken away to reveal the statue.

If you're going to use clay for a large figure, remember that the only thing that needs to be clay is the outer layer. The interior might be hollow. Build an armature in the basic shape, then put clay over the top and sculpt the outer textures. Most earth clays need to be fired, so you've got the problem of figuring out how to heat the thing properly so that the clay cures correctly. Or build it in pieces where each section can be cured separately then joined together for the larger piece.

Hope this helps...

For larger lost wax sculptures you can't sculpt in wax normally, even for small ones it's not really common. They create a positive, from that they make a ceramic negative, pour the wax into the negative to create a wax positive, then use that to create the final negative for the bronze to be poured in. Pour in the bronze, remove the mould (either by smashing it or if it's built in separate pieces pulling it apart), file down the jaggy bits and apply your patina. For more complicated bronzes it may be modelled in multiple parts and welded together.

A popular medium among sculptors is called Magic Sculp.
http://www.magicsculp.com/
I've used it and a similar medium called Apoxie Sculpt.
http://www.avesstudio.com/index.php...ion=com_virtuemart&Itemid=4&vmcchk=1&Itemid=4
I build an armature from wood, styrofoam, and newspaper. Next I add a layer of fiberglass to give the aperture form and rigidity. Then I sculpt over that with multiple thin layers of the sculpting medium.

The media are two-part epoxy-resin mixtures that you knead into a paste. It takes several hours to set up, so you have plenty of working time. After it sets up, you can carve it, sand it, drill it, etc.

Some people develop an alergy to these mixtures after a time. I did after several years of using them and can no longer use any kind of two-part epoxy-resin stuff.

Chris, does he need to be making the life-size sculpture out of the clay? He might be making a small version from clay, with the intention of later making a full size one from something else?

If the piece is going to be cast later it would be oil clay over a wire and/or wood armature. Oil clay cannot be fired, it is designed to be permanently workable. As Drachen Jager says, this is the positive. When it is finished you add sprues and spray it all with silicon or whatever so the casting material doesn't stick in every tiny pit and fissure, then. Now you pour a casting material around it to make a negative, which can be anything from latex or plaster of paris to cement depending on what you want to cast the sculptures out of. If you are going to cast metal or glass you need to use something really strong and heat resistant, but if you are going to cast stone powder, cement, ceramic, resin, etc. then it doesn't need to be heat resistant but may benefit from being a little flexible. Typically the original sculpture is destroyed in the process of removing it from the mold and the oil clay is reused for new sculptures.

What is the piece for?

If he's casting (in bronze, resin, plaster etc), the initial work is usually made of clay and the mold created from that. For something like a face or body, he will begin with some kind of basic frame - I've seen them made of wood with pegs to help support the thicker parts of the clay such as on the cheekbones. The Lost Wax technique is more common in places like India and tends to be used for smaller figurines even today (you know the little metal animals you can buy in "ethinic" shops, thems Lost Wax technique).
The body might be assembled on a frame of plaster bandage covered chicken wire. It would probably not be done in one piece because of the weight of the piece when finished.

A lifesize clay figure (a la the Terracotta army) would be hollow. It would be separate pieces which were assembled after firing.