Your friend is correct in what they say.
However, it has absolutely no relevance to your problem.
Fuel (and other aircraft stores) taken on board an aircraft are free from customs requirements.
In simple terms all he has told you is that you do not have to declare to customs on arrival the fuel in the tanks that you are importing into the country or when leaving, the fuel you are exporting.
It is not the fuel you are having a problem with it is the people on the aircraft and their need for customs, immigration and so on to be in attendance for an arrival.
Once again please give us the location an I may be able to give a more specific answer.
Regards,
DFC