I only use 1g as temp tanks, or for sick fish. Your Mom could keep the 1g, but you'd have to get a larger tank eventually. She could return it, spend a few dollars more and get a bigger tank you won't have to upgrade :)
From one Mom to another... (I'm 27 with two children) please just get the right tank the first time and save yourself some money/headache. I bought 1g tanks, then 1.5g, then 2.65g and now I have 3.3g tanks. If I would have just bought the 3.3g tanks the first time, I would have saved so so much money. 1g tanks are the minimum, and small. Water gets dirty faster and it's more work to clean them. They also have no room for any real decorations. Please remember this is the water that this fish will be urinating and dropping feces in, the more water the better!