Being that I'm a YJ owner and have quite a bit of experience with TJs, there isn't a whole lot more to look for on a YJ than you would look for the same on a TJ. Mainly depends how it's been taken care of.
Rust is a big issue, so definitely look for that. Depending on the year, rust will probably be worse the older it is for two reasons, one being age in general and the other being that they were less galvanized in the beginning and became more equipped with rust proofing over the years. Any of them can still get it, though.
As far as engine and drivetrain go, if it's an 87-89.5 4.2 5-speed, avoid it like the plague unless he wants to buy a new transmission. Those Peugeot BA10/5 transmissions are horrible. If I were to buy one of those Jeeps, the first thing on my list would be to swap that transmission to an AX15.
If it's an 87-90ish 2.5, it'll be TBI which is a very weak engine but not necessarily unreliable or anything. Just very weak.
91-95 2.5 and 4.0 are both as reliable as the TJ engines, just older. Reliability really comes more from being taken care of than anything else. The 4.0 still has the same oil leak as all the TJs, most of that type of stuff never changed.
YJs are okay Jeeps, you just have to get past the quirks of them. They ride real rough (it can get better with more expensive lifts), but stock suspension will ride rough and almost any YJ you're ever gonna buy is going to have a dirt cheap RC lift on it.
Mine ( '94 4.0 5-speed) hasn't ever not started on me, at least not anything that any other vehicle won't do. I've had a bad Interstate battery and a bad starter from O'Reilly. That's it. The worst part about the whole YJ is probably severe lack of original parts these days. I plan to keep mine as a side toy for however long I can keep it running, that all sorta depends on how long there are replacement parts to keep it going. Mopar sure seems to not have enough stock of anything I need anymore, as some sensors take me hours of searching to find. Good luck.
