There are several hydrocarbons you get when you refine oil.. methane, propane, butane, pentane, hexane, heptane, and octane. Octane is an actual molecular substance. Octane ratings for pump fuels are supposed to be the percentage of octane vs heptane in the fuel. Octane handles compression very well - heptane does not. I'm guessing this was actually the system used when gasoline first appeared(when it was just heptane and octane in gasoline..), but today, companies add octane boosters, since its cheaper than using just heptane and octane. Lead was one of these substances, but since the 70s, its been ruled as unsafe for the environment, so its no longer used. The 'with MTBE' you see on pumps is just telling you that there is an octane booster in the gas.. MTBE is an octane booster, with a octane rating of 118. Basically, all the system represents now is IF the gas was actually just heptane and octane, what percentages of these would be required to equal the compression capacity of the fuel. Now octane isn't the best fuel in the world for resisting detonation, but since the system was already in place once these other fuels were discovered or tested, they simply extrapolated to guess what octane rating it would be, if it were possible to have 100+% of octane. They basically derived a formula to calculate octane based on testing different octanes of gas that is pure heptane and octane. The formula we use in the US is (RON+MON)/2. Many other countries simply use the RON rating, which is significantly higher. I think a RON rating of 98 is around 92 (R+M)/2, but it depends on the exact mixtures of gas and what the gas contains. You can have two different gas mixtures that both have a RON of 98, and different (R+M)/2 ratings. If you look on the pump next time you're filling up, it'll say something like "Using (R+M)/2 method" under the octane rating.
To answer your question, I think your friend heard this somewhere, and some info got lost in translation. Both heptane and octane are fuels, they're both flammable/explosive. He's somewhat right in saying there is a percentage involved... there was. Now its merely the average of two tests(RON test and MON test) that are based on the detonation thresholds of the original heptane and octane blends.