If the declaration of the father stated in the Mishna is not valid, the Gemara says that he does it anyway, 'Orchei D'Tsurba MeRabanan LeHadurei'.
It seems that this is an incautious thing to do, since the daughter, hearing of this annullification, is likely to think that her vows have been annulled, and will transgress her vows!
Your question would be valid if the father was being Mefir the Neder on his own. However, since the Mishnah is speaking about an Arusah who needs the Hafarah of the Ba'al anyway, there is no danger.
Also, l'Hadurei does not mean that he actually says he is Mefer what he has not heard. Rather, he simply discusses Hafarah and Nedarim with her, until he finds out whether she has made Nedarim in the past that he can be Mefer.