Hi Snowy,
You need to keep these birds separate for now. She is much too young to be with a male. Cockatiels should not be paired with another unless both birds are at least 2 years old. She is in danger being with an older male. The reason for his behavior is he wants to mate with her, so he courts her, then she doesn’t return his affection because she is not sexually mature. He responds by attacking her. He will get frustrated to the point that he will kill her if you do not keep them apart. She should not be with a mature male because as she gets a bit older, her hormones will get triggered and she will try to lay eggs before she is ready. This often results in egg binding which is usually fatal. Even with Vet care, egg binding can result in death or so much damage to the reproductive system that she can never lay eggs again. So keep them apart until she is 2 years old and then if you are still interested in breeding them, you can see if they get along. Keep in mind that if you turn them into breeders, they will no longer be pets. You can’t handle them anymore if you want them to breed.
Thank you for asking Lafeber,
Brenda