The CrazyGuyOnABike article linked above is a fair few years old, and the route has improved quite a bit. Assuming up to Holne your friend followed that guy's route, then below is an update to the route on the CrazyGuy site.
Above all else, tell your friend to stay OFF the A38 between Exeter & Plymouth. Effectively it is a motorway and as far as I'm concerned, anybody cycling it has a death wish!
The route I mapped (eventually) joins up with part of the Devon Coast to Coast route (NCN27) and is almost entirely in good condition and tarred. From the village of Yelverton it is downhill all the way to the outskirts of Plymouth. Believe me, after some of the hills he'd have faced, he'd appreciate the rest!
http://ridewithgps.com/routes/1266370