You can probably find some ideas on the internet. I have searched before and found several, some of them were temporary pits built with concrete block. You might also check out the other BBQ sites and see what people have. There are some people that have masonary pits in their house that could be adapted for outside use.
I like Dave Linebacks "Wilbur". Dave's thinking is bbq should be cooked over live coals. He used to have a great BBQ list that was dedicated to discussing cooking over live coals. He built Wilbur at his house in Raleigh NC, and also built another one at his home in the mountains of NC. I think he is living full time in the mountain house now.
Here is a link to his site The "Lexington Collection"
http://www.ibiblio.org/lineback/lex.htm
From that link is a page for backyard Barbecue machines.
Here is the link
http://www.ibiblio.org/lineback/bbq/pits.htm
There are several pits there, Several block pits, some masonary ones, some metal pits, some in the ground pits. You can get several ideas there to help you out. Several of these pits can get you to thinking. I like the one that was built with 2 file cabinets. Kinda cool the way the drawers pull out to get the the cooking surface.
Here is the link to Wilbur
http://www.ibiblio.org/lineback/bbq/pits.htm
This is the link to "Sidney R Pig", Wilbur's brother
http://www.ibiblio.org/lineback/bbq/sid.htm
This is a pit where you build your fire on one side, and then shovel the live coals the the cooking chamber. That is the way NC BBQ is done. Well the way it used to be done by most all, as the number of "wood burners" have really dropped.
Here is a pit like Wilbur
http://www.ibiblio.org/lineback/bbq/beast.htm
Based on what you mentioned about the sheet metal and size, your friend may be talking about building a pit like these
http://www.ibiblio.org/lineback/bbq/rufus.htm
http://www.ibiblio.org/lineback/bbq/stasiuk.htm
This one was interesting in the way they used garden pavers on the inside with the scalloped tops to hold rebar to support the cooking surface.
http://www.ibiblio.org/lineback/bbq/bbr.htm
There are some pages on Dave's site that are great. There is one for cooking a whole hog, if your friend is interested in that. Some history of NC pulled pork, speaking the language of southern BBQ, recipes for NC sauces and other things. It's a great resource. I'm not sure if it has been updated in a while, but the info is still great.
With the pit you are thinking about building, if you are planning on using live coals to fire it, the thing to figure out is the distance from the cooking surface to the coals. Most people use a simiar distance, and I think that is 24 inches. That is from memory though, and would be best to research it.
From Dave's fire and fire and smoke page here is an excerpt about pits
The Southern barbecue purist digs a hole in the ground, tosses in the wood, burns it down to coals, and places a contraption of some sort above the coals to hold the meat. Such traditional pits are tricky! The barbecue joints are somewhat more sophisticated. Most have hearths for burning the wood to coals and separate above-ground "pits" with block, bricks, rocks, or metal sides. These "open" pits generally have a door of some sorts for shoveling in the coals. The meat is held above the coals by metal bars or grills. The proper distance for suspending the meat above the coals is the subject of great debate, but heights in the range of 18 to 36 inches are common. A metal or cardboard cover laying directly on, or held immediately above, the meat helps to concentrate the smoke and prevent ashes from falling on the barbecue as it is cooked.
I hope this helps you out some.
Enjoy!
Take Care, ...Ray
