If there is no sound nor smell, she can continue her salah. These are waswas from the shaytaan, and she must try her level best to fight them, or it will lead to OCD.
Have her watch this about concentration in salah:
https://www.youtube.com/watch?v=RtR_XuFz6WY

My daughter is 15 years she is regular in her salah, from two years she doubts about her wudu n feels that she has passed some air. she does not hear any sound or feel any smell but has doubts that her wudu is broken n performs wudu repeately She she can not concentrate on her salah and has not prayed properly since two years please help us by telling that does she have to do wudu repeatedly