If the aviation fuel he brings in is 100 "Research Octane", it's basically what we used to get at the pump as Premium, just as the 100 Research Octane unleaded racing fuel that you buy at the race tracks is. I suspect the av gas is unleaded also, but it might not be. Only thing that is frowned upon about using av gas in a road vehicle is that it doesn't have any "road taxes" figured into the price as normal pump gas does. Many airports will not sell it to a non-airplane user for that reason (tax evasion at a different altitude?).
There are lots of octane boosters on the market. One of the first was 104+ in the 1970s. It's pretty much a drop-in situation that will raise the octane not more than about 3 or 4 numbers at the specified concentrations. As many articles mentioned, it works better with lower octane fuel than with higher octane fuel. It's not a cost effective substitute for buying higher octane fuel in the first place, though. Therefore, it was usually used for racing vehicles.
There's another octane booster that I recall seeing that required (as stated on the can) rejetting the carb richer when you used it. Otherwise, a lean condition under full power could result--not pretty or desireable. It mentioned something like going 3 jet sizes richer.
Always read the side of the can to see what's in the booster too. The better ones will have a more complicated mix than the less expensive ones and cost more too. Some will interact with certain fuel brand chemistries better than others, so it's basically try and see what happens. Overconcentrating the booster in the fuel does not usually add to the octane level either so following the product instructions is necessary. I doubt overconcentration will really harm anything, but it probably will not help either which would seem to defeat the whole purpose of that exercise.