5 Gallon tanks are pretty good for bettas, but 10 would be best for a single betta IMO.
If you want to, get real plants. I have 2 small betta silk plants cause i liked them, they were small, but you can get some other small live plants that will do just fine on their own.
Make sure you get a heater as well. Bettas may be able to live in cooler water, but as it was stated in an earlier post, 77 degrees is a good temp for bettas. I keep mine around 79-81, trying for 80 as much as possible and zip seems to be happiest in that range.
You can honestly do a beautiful 10 gallon tank for your betta and he/she will be soooo happy. 10 gals really gives them room to swim around. Im very avid of the 10 gal because i think that no fish deserves to be in a tank smaller than that. Let them spread their fins and swim!!!!!!!
My mom has never had a fish tank before, so I'm going to ease her into it. I mentioned getting a larger tank and she wants to see how she does with a smaller one first. I am still in the learning process of having live plants, so my mom wouldn't be at that level yet. Maybe one day!
I did order a heater for her as well. I have a bunch of silk plants that I can use to provide cover, and am also planning on buying some other decoration for him to hide in. Hopefully she likes having the tank and will move up in size. I've been trying to convince her how relaxing watching a tank with fish in it can be!