The Ran explains that if the father would to hafara, and the husband hakama, even if the husband would uproot his hakama (by being sho'el on in like a neder), the hafara of the father would be lost. However, the Ran quotes a machlokes between the Ramban and Rambam. According to the Ramban although the original hafara is lost, the opportunity is not lost. The father can simply be meifer again together with the husband and rid her of the vow. However, the Rambam (and Rosh) disagree, they maintain that once the hafara of the father becomes nullified, the opportunity is lost forever and there is no way for such a neder to have hafara. What is the point of argument? It seems to me that they argue whether hafara is an action that is accomplished through a statement, or is it a continuous state that is put into place through the statement. The Ramban maintains that it is a specific action and can be done multiple times. Therefore, even if the first time was ineffective (due to the husbands hakama), he has the opportunity to do it a second time together with the husband. But, the Rambam holds that hafara is a continuous state. It is as if the father who was meifer continues to be meifer at every moment. Nevertheless, the hakama of the husband ruins the ongoing hafara of the husband, therefore the repetition of the hafara by the father doesn't add anything to the existing state of hafara of the father, so once it is lost, the opportunity is gone forever.
Another interesting point that was raised in the kollel: If in fact being sho'el on the hakama removes it as if it was never done, how can it ruin the hafara of the father. Although at the time the hakama was done the hafara of the father becomes unable to combine with the husband to facilitate hafara, but since the husband uproots that hakama as if it were never done, the original hafara of the father should not be effected at all? I think it is clear from the Ran that he holds, although the husband being sho'el on his hakama completely uproots the hakama from his own perspective, it doesn't erase it as if it never existed, therefore any negative impact that it had on the father's hafara which was to nullify it since at that moment it was not fit to combine, cannot be repaired by the father being sho'el on the hakama.