tell him to buy mine.
Mostly the same stuff as e30's just look out for rusting on the rear shock towers(common problem), leaking sunroof, and floor/ frame rust. Sunroof can be fixed but is a pain in the ass. Make sure you drive it for a while and turn it off and start it again. Tons of e21's have a warm start problem which means after driving it for a while if you shut it off and try and start it again relatively soon it will refuse to start until it cools down. Takes a lot of truobleshooting to fix.
How much is the e21 going for?