It's hard to imagine permanent damage to the Z/28. It, of course, is a very high compression engine, even by '68 standards.
Of course, with todays fuels, I doubt if he can make it run right without all sorts of fuel additives anyway. I wouldn't think that there would be any residual diesel, but if he only ran out a half tank before these two refills, he could still have a good bit of diesel in the tank. It wouldn't hurt to put a can of gas in the trunk, run the tank dry, and then fill it up. That's the only way that you'll really know that the tank has been rid of the nasty fuel.
After that, it wouldn't be a bad idea to replace the plugs and give it a good going over. After that a good, hard run through the gears on an open road should clean it out. In the years when I drove cars like that, I was always looking for an excuse to run through the gears. Come to think of it, that may not be a true statement. I never needed an excuse to run through the gears in those days. Of course, we had no short supply of 102 octane for $.30 a gallon either.
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in