Is it normal for a 15 year old for her wisdom teeth to have never grown in?

Yes: Late teens is when you would expect them.

Yes: Are they visible on radiographs? If not, she may never get her wisdom teeth. They are the most commonly missing teeth right after lateral incisors.

Yes, thats normal: It is normal for a 15 yo to not have erupted wisdom teeth yet. They are most likely developing under the gums. They should still be evaluated by an oral surgeon, as it may better to extract them sooner rather than later if they are in close proximity to the sinus or nerves that live near the wisdom teeth. He or she will be able to discuss that with you.