As usual it comes down to condition & buying the best you can afford. Get your mate to contact his local region of The Independent Porsche Enthusiast Club (TIPEC). These guys know the cars inside out & usually work on their own cars.
I've personally owned 3 944s & I've got one in my garage at the moment, they're great cars & as cheap to keep on the road as most old cars. Shop around for parts & use specialist breakers (much like owning a Mk2 Golf!)
The 944 is often thought of as the poor mans Porsche (by people who've never owned or driven them) I previously had a Turbo model & it was one of the best cars I've driven, nicer to drive than my old 911 & more practical as a daily car than my Boxster.
Tell your friend to look out for a 924S as well. They usually sell cheaper than the 944, the sills are less prone to rust & it's got the same 2.5L engine. The one I had had more BHP on the rollers than most of the 944s at a rolling road day I attended.
Engine wise they're pretty much bomb proof, regular servicing like any car is a must. Check the belts have been changed & hopefully the waterpump has been done at the same time.