87 clutch problems

Ok, so my friend has an 87 Cherokee. 2.5 manual transmission, not sure which transfer case or which transmission though. He bought it back in August work some clutch problems. He took it to his mechanic who then replaced the clutch. Ever since he got it back it seems like the clutch never fully engages. The mechanic then said that it was because the pressure plate or the flywheel that he replaced was too thick and that he should drive it looks that and it looks get better. I wanted top call bullshit when he tells me this but I bit my tongue and didn't.

So I guess my question is, what could ACTUALLY be wrong with it? My first thought was the slave cylinder? I've read a lot of threads and it sounds possible and then I read that in some years the master and slave cylinders are one piece. Is this on of those years?

Sorry so long and I might've missed some things buy just ask:read: