I would not be so worried about explosion.but u can burn down the house.let it dry out by wiring the blower on the gun to run.don't forget to shut off the fuel. Tell customer u understand they did not know what they did added to the prpblem.tell them how lucky they were that it didn't light after they soaked the combustion chamber with fuel.it really can set a house on fire.good question
I've never had an explosion happen before but it can make an awful loud bang and the draft regulator may go flyng across the basement. If I were you I would open some windows fire it off and let it burn off itself it usually takes about 5 minutes, don't worry about the rumbling or smoke it will subside pretty quickly I've done it many times.
Do not allow an accumulation of vapors , spider webs can cause a delayed ignition that resembles an explosion from the house, I have seen boilers do this, luckily I was not in the room that had its steel doors blown off the hinges!