Here in NZ, and Im sure else where in the world that unused and old Avgas (there is also an expiry date on it) is collected by the oil companys and repackaged into 20L drums to be resold as 100+ RaceGas.
Running to high an octane fuel in your car will not generaly have any negative effects.
Your friends over heating is more likely a result of having a cooling system not able to deal with a heavily modified engine, and or from running to much ignition advance.
The only reason an engine orginaly designed for the streat won't run on avlaible fuels is if its been modified, or is running to much advance.
AvGas does contain lead, and if you run it in a car with a catalytic converter it will clog it very quickly.
Lead is also not good from some types of O2 sensor.
And note that in most countries that have totaly converted to Lead Free petrol I think you will find its very illegal to run leaded gas on the street, this includes AvGas, and here in NZ even RaceGas is illegal on the street.
Don't go rushing off to your local airport expecting to be able to get free fuel.