You don't say whether your friend will be outdoors in Texas or North Dakota (assume Texas), hiking distances, car camping, or hunting camps, or how much clothing he already owns. But he's signed up for a 'wilderness survival course' in four months which sounds aggressive to me for someone just getting back into outdoor living from behind a desk. Possibly the organizers of that course have a pack list that will guide him in purchases. Most wouldn't want to support a camper without a core of gear. Anyway, in order of priority:

Boots - buy what fits. That soaks up $140-200 of your $600 budget.

Pants, shirts, socks, underwear etc - a survival course calls for moderate to better quality wear, so careful shopping for the usual convertible pants, wool and polypro stuff will net you another $75-125. otoh any longish survival course calls for 4 pair of wool socks, eating up another $60-80 of your budget.

Waterproof layer - you may find decent deals at Campmor, Sierra Trading Post, Backcountry Outlet etc. REI's February sale is just ending, there were some good in-store deals on Marmot Precip gear. Or go surplus and buy a heavyish poncho.

Pack - again, if he's gonna carry this stuff along a trail, go for what fits him best and fits his gear. Probably $150-200 again. Best cheap light sleeping pad is a traditional blue foam or Thermarest Z-Rest.

Tent/bivy - he should take his lead from you, if the environment permits it go light and tentless. I would avoid the pricey goretex though and save on a more basic bag cover, like the Montbell Dry-tec ($130 vs $180 for the goretex version). Has worked great for me for 3+ years, dry inside in snow or rain.

Sleeping bag - this seems to be a religious issue with hikers, you're better off going in store and trying some out, then choose the cheapest (and second most comfortable). When it comes down to it most modern bags work great in most weather, which is what I think your budget allows. Figure $150 for a moderate priced bag, probably not down.

I think we're at $600 and your friend is half dressed and somewhat exposed to the elements, without a warm layer, and without food or a way to prepare it, or 8-9 of the essentials. Shop garage sales, eBay, Craiglist etc to make your budget goal.

I guess this kind of question is what the backpacking gear forums are made for, but usually you aren't trying to outfit anyone from ground zero or on a low-moderate budget like $600, and not with a wilderness survival course or adventure looming on the calendar. My own summertime hiking gear weighs in at 20-30 lbs depending on frills and food, but it was bought over time and mostly on special. You'll get lots of advice about living under an issue poncho and eating MREs etc, which may make fiscal sense but I'm no judge of your friend and what his preferences may be. He could take to sleeping under the stars or feel better in a tent. When all is said and done he will a. borrow alot of your gear just starting out and b. maybe model himself after you and whoever he hikes / camps with. And if he goes on a real wilderness survival experience that will whittle down his priorities to things that maybe he wouldn't purchase right off.

Get him out now, this coming weekend, without his gear of choice, sharing your gear and experiences. See how often he can fit being outdoors into his desk schedule, that will decide if he's ready for a survival course. He probably has some cobwebs and a few pounds to shake off before he's ready. I guess I'm pretty leery of throwing anyone into that bathwater straight off.