I have camped a few nights in my '02 WRX Wagon. Main question is: How tall is your friend (and his lady)? That will limit sleeping arrangements.
I'm 5'6", so here's what I did:
Slide front seats forward and incline them as far forward as possible. Flip the rear-seat headrests around backwards. Now drop the seats. The headrests are now in a position to support a board to extend the bed area slightly. (Mine was 3/4" MDF about 8" wide by the width of the car, IIRC).
Using a cheap (Coleman?) twin size inflatable mattress, it fits between the interior humps for the strut towers. Inflate the mattress and wedge it down in place (tight fit.) You'll need the air mattress to smooth over the floor: In particular the backs of the seats have a weird "lip" that sticks up when folded down.
Fill up the corners with your soft luggage (clothing, etc.) anything hard will have to go on the roof if it doesn't fit in the front seats. With the air mattress, headroom was "cozy" but livable. You won't be sitting up in bed (comfortably), but you don't be whacking your elbows on the roof when you roll over either, which is better than I can say about some places I've slept. It WILL be cozy for two people - hopefully they like each other.
I sewed a set of curtains that stretched from the front visor mounts down the sides and across the back windows - nylon cords sewn into the top of the curtains hang everything nicely. I used S-hooks to catch the front visor mount and the rear strut mounts at the top. A little hook-side of velcro sewn onto the bottom of the curtains has just enough grab with that weird interior trim that Subaru uses in the hatch area to keep things secured at the bottom. I might have also used S-hooks to attach the bottom of the curtains into the front dash air vents, can't remember for sure. Combined with a regular front window shade it was more than dark enough to sleep, even when sleeping in a lighted parking lot (MakerFaire '08, for example.)
One nicety: I was able to "hack" the rear hatch release with an extra "pull" so that I could open the hatch from inside the car, making ingress/egress a lot easier. Find the mechanism and attach a thin pull-cable and run it out through the hatch trim.
For winter camping - Get a good sleeping bag, the car won't offer enough insulation by itself, but it'll keep you out of the wind. In humid climates you may need to crack a window to avoid massive condensation (trade against temperature concerns.)
Hope that helps.