[Nickj]

No adjustment on the clutch rod, all you get standard is minimal adjustment at the lever.
If the screw in the lever has been moved it can shift the piston to a point where it pumps up pressure but doesn't release it, not the case here though.
If the seals at the master end have gone then he should be able to feel it, pull the lever in and it will get 'softer' as the fluid leaks back past the seals. Some can even leak out.
If the seals at the slave go you get the same effect and a leak of fluid at the slave.

I'd go with your idea, give it all a good clean out then refill and bleed which won't do any harm. While the slave is off check the push rods are straight and not too worn, they won't be.

I'd also think making sure the friction plates aren't sticking at the clutch basket itself. If it's just been sitting and not started it makes this the most likely cause of his problem