it is not ideal but can be done If you monitor the water stats Daily and water change as soon as problems occur,
the heater doesn't need to be switched on whilst the goldfish are present, or you can keep it at its minimum temp which should be around 22C
Although Fishless Cycling would be much better if you can convince her to choose that method instead
Edit: if she already has a tank she can just take 1/3rd of the filter media across to the new tank allowing her to start off with 2-3 hardy tropicals and then very slowly build from there