I really think bettas are quite content in a gallon container. I wouldn't put them in anything much smaller. My three males are quite happy in theirs, sitting on the kitchen table where they get plenty of attention. I've had each of them in a ten gallon (at separate times of course) and only one of them seemed to like it ~ the other two seemed nervous, like they were afraid another fish was going to come from the other end of the tank and fight them.
Females on the other hand are fine in a ten gallon. I have two girls together in a planted ten g and they're happy as clams.
I also completely
agree with putting plants in there with them, just not anything that covers the top completely. They must
be able to come to the top to get a breath to fill their labyrinth organ or they'll die. Yes, you really can drown a betta!
Java moss is a GREAT plant for a betta tank, as is Java fern. Keeping plants in with the bettas not only helps keep the water clean of course, but it also gives them somewhere to rest and just makes the whole thing look better imho. If you're going to get a crowntail betta, then Java moss isn't a good idea ~ the strings or "spines" of their tail gets caught in it quite easily, tearing up the fins and/or getting them just plain stuck to where you have to get them untangled. Java Fern or anubias would be a better choice for them.
The best food for them by far imho is Hikari Betta Bio Gold ~ just three or four pellets per day is plenty. Frozen bloodworms every now and again as a treat is a good idea, too. She should also keep some frozen peas on hand in case of constipation ~ just thaw one out, take the shell off it, crumble it up very tiny and feed it. But if she feeds only lightly and skips a day every now and again (no more than once a week), she shouldn't have any problem with constipation in the first place. Just tell her to look at the fish from the top every now and again to make sure he's staying full throughout his body and adjust his food accordingly. It's hard to tell if a betta is starving from looking at him from the side ~ but it's easy from the top.
I find that mine don't need heaters either. I keep my house pretty cool and they're just fine with it. Same thing with a filter. The plants will take care of that ~ that and water changes every week or two. If you have plenty of plants in there and a tank of atleast a gallon, you really shouldn't need to do it more than that. Changing it every couple days will stress the fish out, especially if you have to add a water treatment, so I wouldn't recommend it.
Some bettas like a buddy in with them. I have one who loves his apple snail (mystery snail ~ the smaller ones) and gets upset if I take it out. Yet I have another one who could care less about snails. He's still young yet, and still plenty agressive. The old man is the one who likes the snail and also happens to be the one who doesn't mind a ten gallon tank ~ that's probably because he's not so agressive anymore, now that he's aged somewhat.
I'd recommend getting the betta at a Petsmart if you have a good one near you who keeps their tanks clean and bettas in the filtered "barracks" (a long row of small, 5"x5"x3" or so individual "tanks" so-to-speak set up above the other display tanks, at eye level, that have water running through them). It's not the best thing to have them all hooked up to a central filtration system like that since obviously diseases can spread easily, but it's a far cry better than keeping them in the cups. As you probably know, the bettas gills get burned terribly from ammonia in those cups, so that shortens their lifespan. Even if you find the perfect one in a cup at Petsmart, it's still better than buying one in a cup from a petstore with less traffic ~ Petsmart sells them so fast that the ones you see on the shelf haven't been in those cups long. I haven't had such good luck with bettas from small petstores and I think it's for this very reason. I just lost a gorgeous crowntail a couple weeks ago that I knew better than to get ~ he was in a cup at a small petstore and I couldn't resist. Three weeks later and he was belly up.
Good luck! I think this is a thoughtful gift and that your "MIL" will love it! Bettas have quite the personalities!