'91 Miata oddball clutch issue

My nephew bought a beater '91 Miata for $500. Among the expected issues of a $500 Miata, it has one weirdity I don't recognize:

The clutch only works when the car is cold. When it warms up, it won't engage.

It sounds to me like a hydraulic issue rather than a mechanical problem with the clutch itself. He was planning to replace the clutch to see if that helped, but I figured I'd ask here before he spends that time and money.

I haven't seen the car; this is just what I got from him by asking about it.

How does the slave cylinder look?

Along that line, how does the line connecting the master & slave look? It could be closing up as the fluid heats up. I've run into issues with really old/bad brake lines, not with a clutch, but it's the same hydraulic action..

Is the fluid dark?

Simple way to determine if hydraulic or mechanical: two bolts hold the slave cylinder onto the car. Remove them. Warm up the car... If you still don't have engagement, you need a clutch.

Simpler thing to check: can you push the slave cylinder piston back into the cylinder? It'll be a bit slow, but you should be able to do it.

If not, it means someone's adjusted the free play out of the clutch pedal and covered up the return hole. As the fluid heats up, it'll start expanding with nowhere to go - and that means it'll start pushing on the release arm.

When you say it won't engage when warm, do you mean not at all, or it slips?

Haven't seen the car, so I can't attest to what the fluid or lines look like. If you can remove the slave cylinder and still operate the clutch, I'll tell him to try that before he drops the transmission (or pays somebody to do it). Thanks!

Clutch itself won't be temperature sensitive. Best and easiest thing to do is replace the Master and Slave and maybe add a braided flex line. The master/slave usually fail in short order of each other. Thats a hell of a lot easier than dropping the trans.