That would get him started. But I find my Tokina 12-24mm wide angle zoom VERY handy for interior shots. Able to capture a lot more of the room in a single shot. I think he would be better off with a D40 or newer used body & a wide angle zoom.
Definitely go for the wide angle lens here. A Sigma 10-20 is a great place to start especially if the interior rooms are small. As for the camera, get the cheapest one with Live view, this helps when you are taking a shot from the corner of the room and you cant see through the viewfinder, trust me it makes a world of difference. If he can stretch the budget a bit further get the D5100 with the articulating screen.
Then your friend can start saving for lighting. In the meantime, The LR/Enfuse plugin for Lightroom will be a nice, NATURAL, job of stacking multilple exposures. You get properly exposed windows AND interiors. At the same time.
Your friend should be thinking about a lens for real estate as well, where wider tends to be the order of the day. I just picked up a Sigma 8-16mm I'm dying to try out on my D7000 (or D300). From everything I read, it's slightly better than the Sigma 10-20 (which I used to own) and should give some nice perspectives, especially for interiors.
To answer your original question more on-point, the D3100 and 18-55 would, IMO, barely enable the type of photography your friend will need. My recommendations (well above his budget, for sure) would be:
- A camera with low light capability: the D3100 is okay, but the D5100 may be better
- A SWA lens (like the 8-16 or 10-20) along with a more standard mid-range zoon (16-85 would be my recommendation)
- A flash he should bounce or use off-camera for some interiors that have limited lighting
- And a tripod for those slow shutter interior shots
If he wants something quicker and dirtier than that, have him get a m4/3 or even a P&S camera.