Guppies in stillwater?

Hi my sister recently had her betta die of old age and would like to get a new fish. She kept it in a 2 gallon tank and was wondering if she could keep a pair of guppies in the tank. The tank has no filtration or air supply as the betta was a surface bleeder. I have heard of gupies surviveing in "stillwater" tanks but i was unsure. Please help me.