If she is sexually mature and shows interest in spawning with the male, it shouldn't be a problem. IF she is too young or not sexually mature and not showing signs( that first egg spot) he will most likely kill her if he's ready to spawn. This is one of the hazards of breeding Bettas. It needs to be done specifically and not just "willy nilly, throw a male & female together and see what happens" kind of thing. There is no guarantee that the female will survive the spawn. This is why I only breed the fish I don;t mind losing.
Hope this helps