disc brakes for 85 lada niva

Are you serious? I suspect your mate is complaining about the lack of rear brakes very typically of Nivas...gotta keep adjusting them. Replacing your rear shoes more often is a much cheaper option than going for discs. Know a mate who has a Hilux rear diff in his Niva. Think it still has drums but better brakes/bigger axles/after market locker availability (don't think you can get a detroit for a Niva anymore). Talk to Grant at Lada Autosport, Rocklea Brisbane if he's really serious.
Ignore the expected comments of "why bother, it'll cost more than the car is worth". If your mates got a good one, hang onto it, they're good little trucks when well maintained and although no rock hopper, will mix it with all the bigger ones for normal offroading at 1/10 the cost.

I would have your mate committed.......

sigh! - one mans nightmare is another mans wet dream!

I have a mate with a disk-brake Niva...

It's a 89 model, he's got a cut-down Mercedes autowagen (4x4) axle in it. It is sprung off the same spring mounts and such.

I personally wouldn't bother, just keep adjusting them and such. No biggy and you will introduce too many problems into what is a problematic rig anyhow.

Thanks guys for your responses. He believes there is some way an Escort system may work but will take on board what has been given.