If the matter is as you describe, then your friend has to tell her fiancé about this defect, or one of her relatives must tell him, so that he will have full knowledge of the matter, because not telling him is deceiving him, and whatever Allaah has decreed will happen. No one knows where goodness lies. We ask Allaah to bless her with a righteous husband. 

If the husband finds out later on that his wife cannot hear well in one of her ears, and that she was suffering from that before she got married and did not tell him, that may upset him and he may regard it as deceit, and he may hate his wife because of that. So she has to tell him now, hoping that the marriage will last and so as to set things straight and so as to ward off any negative consequences. 

The Standing Committee for Issuing Fatwas was asked: I have a sister who is affected by the evil eye, and someone has proposed marriage to her. Do I have to tell him that she is affected by the evil eye? If I do not tell him, am I regarded as deceiving him? 

They replied: 

The guardian (wali) has to explain to the one who is proposing marriage any defects or sicknesses that are present in the woman concerned, if the man is not aware of that, so that he will have full knowledge, because not telling him about that is deceiving him, and the Prophet (peace and blessings of Allaah be upon him) said: “Whoever deceives us is not one of us.” End quote. 

Fataawa al-Lajnah al-Daa’imah (18/62). 

And Allaah knows best.