Check the drainage holes for the roof - they feed near to where the back of the sills (where the door shuts)... If they haven't been cleared and rust appears expect some rust. People do repair panels for the sills.

Also, arches rust quite heavily too if the shit doesn't get cleaned out of them.

Engines are generally bullet proof (they're cambelt happy - if it snaps, time up and away you go) but check for servicing. Coil packs and cam/crank sensors can go but at the end of it, its a car - you buy, you enjoy, you fix, you enjoy. A lot of people drop super chargers on theirs.

I had a 1999 MX5 MK2 (1.8)... She loved to scream! I'd go for a MK2/MK2.5 if I was your mate, refined over the MK1 but not as expensive as you think.

The prices are coming down soon as winter is glooming - the prices start from £500 or less for ones that need work and stretch up to £3-4k for say a MK2/Mk2.5... Tell him not to settle for the first he sees, theres so many on the market he could get a real bargain. Hard Tops are worth ~£300 so if he gets one, thats money hes already saved.