Maxy makes good points. When they are not there, they need to make sure the behavior isn't being rewarded. Gate off the rooms he uses the furniture in. If this is not possible, they need to crate him when they are not home.
They need to teach him "on" and "off". These commands/cues/tricks are necessary for the furniture sleeping, but are useful at other times as well. Point to the couch, tell him on. If he doesn't get right on, use whatever "cue" they've used before when they say on (pat the cushion, for instance). Praise and treat. Leave the room, get dog to follow. Turn around, go back into room and repeat "on". The dog should understand and respond to "on".
The next step is to teach off. Point to the floor in front of Rover's spot and say "Rover, off". His eyes should follow the finger, drop treat on floor. If he's big enough to get the treat without getting off the couch, fine, but each time you repeat the cue, drop the treat a little farther away so he has to get off the couch in order to get it. Praise each time. On. Off. On. Off. Repeat over and over. For treats, use his kibble, bits of meat, cheese, etc. Make it interesting. Once he has pretty well mastered on/off, use going outside as the treat instead of food sometimes, or a chew toy, or a game.
As Maxy suggested, give him a cushy mat to lie on. I like mats because they are portable. Once he has mastered the on/off, praise when he gets off, but go to the mat and point to it, drop treat. Use, "go to bed", or "Rover, mat", or "Rover, blankie", or what have you.
Ordinarily, you would phase out the "on" cue and just use off. But, a better idea is to use "on" for getting in the car, etc. Again, repeat it over and over, having him get into a back seat as well as through the tail of an SUV, pick up, and station wagon. Take a couple of trips to the vet's office to practice good manners in the waiting room. While in the examining room, have him lie down and roll to his side. Allow someone to examine him, then have him roll over to his other side for exam. Your vet will love you for this.