Have you tested the water yet?
Maybe show them the possibilities of what fish could thrive in the tank, not just survive.
Teach them a bit about a goldfish's requirements, hopefully they'll change their opinion.
I recommend re-homing the goldfish and either getting some small temperate fish, or go with some community tropicals.
Minnows, Platies, Guppies, A single Betta, and Honey Gouramis all spring to mind as possible species for your tank size...