Well, it's kind of puzzling how he was getting better than normal mpg then all of the sudden things went into the tiolet - big time! Something had to have changed to make it do that. Timing is fixed, so I doubt that's it. It sounds like the carbs have been checked and everything's copecetic there. He's not complaining about any power loss, so that seems to eliminate mechanical troubles like a cam chain that's jumped a tooth, poor compression, etc. It's a real head scratcher from here, which is why I hadn't posted any troubleshooting tips - I just don't have anything to add.
I guess if it were my bike, I'd start with the basics and start checking everything. Make sure the cams are timed right, valves adjusted right, the timing components are all where they should be, the plugs looks good, compression is up to spec, air filter(s) clean, and so on...