A 2.5 would be too small for a goldfish. :( A betta or some some snails or shrimp is all you can keep in a 2.5. The bulging eye ones are very particular and you have to be very careful with decorations so they don't burst the eye sacks. The minimum for fancy goldfish (like Olympia's and thekoimaiden's) is 20 gallons for the first goldfish and another 10 gallons for every goldfish after that. Slim-bodied goldfish (like littlebettafish's shubunkins and the "common" or "comet" goldfish many people keep in bowls) really do best in a pond since they get so large. Maybe she could try a different kind of betta? Or a different gender? Or make the tank into a natural planted tank with soil and live plants? I did this to one of my tanks, and it is very inexpensive. Since you already have the tank, you could get some topsoil ($4 for a large bag), use the gravel you already have, and some plants that don't need flourescent bulbs such as hornwort and water wistera ($4 for a bundle of each at petsmart). I got a bundle of wisteria, hornwort and moneywort and it was enough to fill a 5.5 gallon, so you should have more than enough.
10 Gallon Tank:
~Countless Pond Snails~1 Espe's Rasbora~1 Harlequin Rasbora~
5 and 5.5 Gallon Tanks:
Avatar by Fenghuang
Many of my scaly friends from my original tank from 2011 have passed, except for two! I am in the process of re-doing my tank. :)