You are not condescending. I made this question controversial on purpose. On the Wikipedia page on Weight you can see 2 definitions:
Gravitational definition:
The force acting on the object due to acceleration of gravity. This force is given by the formula W = mg where W is the force, m is the mass and g is the gravitational acceleration. The mass of the cat with the helium is greater and there for, that group of objects has more weight.
Operational definition:
The force measured by the operation of weighing the object. If we assume the game doesn't take place in a vacuum, the helium balloon would exert an upwards force on the cat and thus the scale would measure less weight.