I've never heard that one before.
Any hydrocarbon fuel, gas, diesel, kerosene, wood, coal, propane, natural gas, etc., will produce CO when burned. The amount of CO produced will depend on the amount of oxygen available and the efficiency of the combustion process. Complete combustion will give you CO2 and water, which won't do you a whole lot of good either if it depletes the oxygen supply in the room. As the available oxygen decreases, the amount of CO produced increases.
I don't know about a gasoline engine producing twice as much CO as a diesel engine but I will guarantee you that diesel exhaust will kill you just as dead as a gas exhaust. The same goes for natural gas, propane or kerosene heaters used in a confined, airtight room. As they deplete the oxygen, they produce more CO and you go to sleep for a long, long time. Wood and coal fires are slightly different as the gases exhaust up the chimney but a cracked chimney or leaking stovepipe can allow CO to seep back into the room with the same effect.
If he's a friend that you value and want to keep, don't let him put his theory to the test.