I would definitely go with 3 gallons, 2.5 can sometimes be a little unstable. All tanks will hold a cycle if given a filter but the bigger the tank, the easier it is to manage which is just plain easier in my opinion lol. But a 3 gallon would hold a more stable cycle than a 2.5 so it would be easier for your cousin not to harm the fish, you know? That tank is fine but I've heard some uneasy reviews about the bubbler column in the middle.
Something you should keep in mind is looking around your town as well, I think it's called Kijiji, the site where you can find used stuff that people are selling around. But you could probably easily find a cheap used 5 gallon that she could use, might actually end up being cheaper if you plan it right is all. Not saying that you have to, but if you keep your eyes open you can find some pretty awesome stuff that people are sometimes willing to just throw out!
EDIT: Oh derp, don't listen to me and my thing about the bubble column, wrong tank my bad! I looked at it a little too quickly lol