that is not nearly enough space! i rescued a pair of oscars and a pleco from a 5 gallon tank- not each in 5 gal but all 3 in one 5 gal... and the oscars were both around 4 1/2" and 6" and the pleco was probably 6"... it was sad but i put the oscars in a 68 gal acryllic tank. it was on the small side but they were much more comfortable. when i get another oscar set up it will be be 150 gal or bigger.... they require alot of space to be happy... even a 55 for a single fish is pretty small, especially for a mature fish. theyre dirty fish so they need alot of aquarium maintenance- big filter systems, constant vacuuming, frequent water changes, ect...
maybe instead of going after her like a crazed peta member, you could give her some facts on oscar requirements and suggest alternative fish species for her to keep that can be kept in smaller quarters. most people arent attached to their fish like all of us on here and when they hear they can trade a single fish for many that are easier to care for they will jump on it.
Bart (dark red/blue VT)
Schneider (blue/pink VT)
Duster (multi color Double Ray CT)
Croix (blue dragon HMPK)