where would the mass of an 80 kilogram person be greatest
Unlike weight, mass stays the same no matter where you are. Therefore, the mass of an 80 kilogram person will be 80 kilograms here on Earth, in space, in the Moon and everywhere else.
However, if you consider theory of relativity, we know that the mass of an object (in this case a person) becomes greater as the objects speed approaches to the speed of light. In fact as the objects speed approaches to the speed of light its mass approaches to infinity. Hence the mass of any object would be greatest when it is moving at the speed of light. However that is impossible because moving an object of infinite mass would require infinite energy.
Once again, mass does not depend on where you are but rather on how fast you are moving.
Mass of a body or a thing is constant every where in the universe . Mathematically mass is an independent value , but weight of a body or a thing depends on mass and acceleration due to gravity on the surface which we are on.
So coming to the question you have asked,
The mass of the person is approx 80/9.8 = 8 kg
His mass is the same every where in this universe and does not change.