Is it a case of go for the later Mono with the electronic ignition.
True R65's are 1978-1984 with the 81's and up you get lots of new goodies, nikasil cylinders,electronic ignition improved clutch, lighter flywheel, sexy square master cylinder, etc. The 1980-1984 bikes had bad valve seats and it will be a plus if the heads have been rebuilt with the later 1985 and up seats.
My opinion with the later mono shock bikes is, you just have a R100 with a dinky motor. Not gaining any nimbleness with the shorter bike. If your gonna get the BIG bike get the BIG motor.
The best advice i would give him is, buy the best, nicest, cleanest, unmolested bike he can afford. It will by far be the cheapest for him in the long run( and maybe even the short run)
( for all you mono shock guys, I have slapped myself and tossed a brick in my face already so you won't have to)