Normally male and female bettas cannot be kept together unless it is a large tank with many hiding places. Breeders only put the females with the males to spawn and then they are taken back out in case the male tries to kill her. I can't say this is always the way it happens, because there are always exceptions to fish and what they will do. It does sound though like your one male is showing this behavior, and if he doesn't kill her physically, he will probably stress her out until she dies from the stress. If you can keep that hurt female by herself, she should heal quite nicely. Keep your eyes on the other couple as well.
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