If you go to PetCo, don't get the babies, they need more work then the adults and are more fragile. Not a first time pet IMO. A 5 gal might be a little bigger then what her mom expects I would think. A 3 gallon will work nicely or a 2.5 but you will need to do more frequent water changes. 50% every 2-3 days. I'd recommend getting a ghost shrimp or 2 to help eat leftover food and only feed one pellet at a time, like 1 by 1 so you make sure he eats them and get out any uneaten. The shrimp only need food 1x every week so just drop in a couple of crumbs from a bottom feeder tablet. I like to get my bettas either a) a nice hammock that is smooth like this one: http://www.petco.com/product/119651/...f-Hammock.aspx
for them to rest on at night so they are near the surface b) get a smooth decoration that comes near the top for him to rest on
I agree that no rescuing for now although AQ (aquarium) salt and Epsom will be good to have on hand in case of emergency.