Two bettas would only work with a divider, which is not conducive to an attractive tank. A sorority of five to seven female bettas may work, but sororities tend to be somewhat stressful (female bettas can be as irritable as the males) and thus might not be something your GF would appreciate.
Have you considered Pygmy sunfish? The males of most of the species are a beautiful black and blue color when breeding, they breed easily, are quite sturdy (they do not need heaters and are not particularly sensitive to nitrates), and like planted tanks. The one caveat is that they are picky eaters who need live or frozen food, but live food also denotes baby ramshorns and pond snails in this case, so it's not a huge problem.
Have you considered Pygmy sunfish? The males of most of the species are a beautiful black and blue color when breeding, they breed easily, are quite sturdy (they do not need heaters and are not particularly sensitive to nitrates), and like planted tanks. The one caveat is that they are picky eaters who need live or frozen food, but live food also denotes baby ramshorns and pond snails in this case, so it's not a huge problem.