Nikon has gotten away from the idea that more MP's are better and have dealt with it accordingly, although they created the D800 large format sensor with 36 MP that is friggin amazing! Canon realized this years ago and has produced some great cameras, but I am a Nikon guy and will remain as such with the thousands I have invested in gear.
Pete, the first question I always ask is what is he willing to spend? I have a friend who is currently using a Nikon D90 and has branched out into the portrait realm and she loves it. But, she understands that camera is only going to get her so far. Unfortunately, the price range at this time keeps her from going forward with a new body. If your friend is genuinely serious about getting into this business, he should move away from the DX format and go Full Frame (FX). You can still do well enough with the DX...Nikon, Canon and Sony have made cameras that are very user friendly (relative I know) in the DX format and I have made a lot of money with DX format cameras, but they are all beginning to take advantage of the technologies available and making cameras in FX format that are cheaper.
From the info I have gathered from you Pete, I would strongly suggest your friend bite the bullet and get the Nikon D600. While he doesn't need the video, it's there and the camera is pretty nice. I have used one and while it is not quite what I need, it should serve him well. Unfortunately, then the lenses come into play. And that is where they will spend if they really want to go beyond simply being a novice and becoming a pro.
Here are 2 lists that I will suggest in both DX and FX format for your friend to consider if he wants to stay with Nikon. But if he is considering making a move and has to buy camera bodies and lenses, I suggest switching to Canon and going full frame.
Nikon 35mm f/1.8
Nikon 70-200mm f/2.8 VR
These 3 items should run about $2500 tops.
Nikon D7100 - It is Nikon's best DX camera right now. The D300s is the last of the DX format "prosumer" cameras Nikon produced and they are nice...I used one for years, but the D7100 is the best DX camera they offer new. If your friend wants to buy used, a D300 is nice but try to find a D300s instead and these should be cheaper than the D7100. Again, it all depends on cost.
And there are at least 2 lenses they should have: Nikon 35mm f/1.8, Nikon 70-200mm f/2.8. I would also suggest a wide angle, but these 2 lenses will provide all they need to get started in portraiture photography. If they want a zoom lens, then I would suggest the Nikon 17-55mm f/2.8 instead of the 35mm, but for the price, the 35mm is perfect and all you need to do is back up a little or move forward a little. I would still suggest a wide angle lens, but that is not imperative. Group shots will be difficult with the 35 on a DX format because is it really 52mm length. If they want, they can find a prime 20mm for a few hundred bucks instead of spending almost 1000 on the 17-55 f/2.8 they can get 2 prime lenses for far cheaper. Just a matter if they want to switch out lenses.
For the price, the Nikon 35mm f/1.8 is awesome and affordable. The best part of it is that is also will work on DX and FX format. If he can swing it, the 50mm f/1.4 is super sharp and only a few hundred more! I have this lens and it takes great portraits.
The 70-200mm lens is ideal for getting candid shots and can be used in sports action photography as well. I actually shoot casual family portraits with mine. He should be able to find the VR version of this lens used for about a grand, the VR II is going to run close to a thousand bucks more.
I strongly suggest him moving to FX. Most professionals shoot FX and if he wants to be competitive, he should do the same.
Nikon 35mm f/2.8
Nikon 70-200mm f/2.8 VR
This should run closer to $3k.
Again, I would suggest that any pro go with either of those setups starting out. Unfortunately, there are a lot of people calling themselves pro because of the gear so if you don't keep up with them, you better have some skills.
My friend, I mentioned earlier, started out with the D90 and a 35mm lens. She does a good job and has a great eye for the biz. Your frined can get their start with that setup. I would suggest getting a better rig to start, spend the extra...a few jobs and it will be paid for and they won't need to upgrade again in 6 month or a year... To begin with, the D300s (Used) with a 35mm f/1.8 could be enough if money is tight.
One thing I should mention is that DX gear can be found used right now on the cheap, but stick to the sites you mentioned. I have had great success with Craigslist, but I know exactly what to look for. I would never buy used body on Craigslist...lenses only.