I've kept bettas in 1 gallon filtered tanks. Even though it was cycled, 50% water changes every 2 days weren't enough to keep the water clean enough. Lost a lot of bettas to fin rot before I learned better.
I moved up to a 3 gallon, filtered at first. Then I planted it and took out the filter and it stayed cleaner without the filter.
Water lilies are too big. I'd suggest at minimum a 3 gallon (look up 3 gallon rimless tanks for inexpensive, centerpiece-worthy tanks often for under $40). And a lot of plants!! Crypts, maybe swords, Java fern and anubias if it's very low light.
Floating plants are key and can give that "lily pad" feel she wants. The floating plants are what clean the water the most. I like penny wort (weave some stems together and I'm time it looks like lily pads).
5 gallons, or more, with a filter, would be much much better. But if she's determined ... And all the stores reinforce that her idea is fine ... A 3 can be ok if she's dedicated.
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