Have him get the newest he can afford. That being said, I bought an 82 GL1100 for 1200.00 had 29k on it. Ebay became my best freind....I ordered an old set of carbs, practiced rebuilding them, then rebuilt mine. Rebuilt the brakes, actually went over the bike with a fine toothed comb. I had to since the local dealer would not touch it, they said it was to old. Once I did all the work to it I rode the heck out of it, all over. Most fun I have had till I bought the 1800. I ended up selling it for 2400.00. I hope the guy that bought it is having lots of fun with it.
The one big difference I noticed between the 1800 and the 1100 is the turning. The 1100 wants to righten itself and would fight me in the turn. The 1800 goes into the turn and pretty much is hands off, no effort to negotiate a turn.
In either case I doubt he will be disapointed. Their is a 88 GL1500 in Martinsburg Wv for 7k.