I'm afraid that oscar will grow faster than you will be safely able to house him in that 10 gal. tank. They can grow as fast as an inch per month, and are huge waste producers. You will be changing water more than once a week just to keep the ammonia, nitrites, and nitrates down. If you keep him in that tank for a year, his growth will be stunted, and he will not be healthy, sorry. The general rule is to purchase the right size tank for the adult size of the fish when you first buy the fish. Things happen, and that bigger tank may never get bought. If a 10 gallon is all that he can be housed in for a year, I'd recommend that your friend take the little cutie back until a bigger tank is accessible, and then stock the 10 gal. with a betta or something suitable for that tank.
