All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.
The man who knows that he cannot satisfy the sexual needs of a wife should marry only a woman who accepts him despite this shortcoming because the ability to have sex is a precondition to marriage.
But if this inability or frigidity happened after the marriage because of illness or because of other things and if this frigidity is complete, then the woman has the right to ask for divorce, although being patient with her husband is better if she can do so. But if she fears to commit Zina (adultery), she should ask for separation as to stay chaste and to protect her dignity and honour, and to be free to marry another man who can help her remain chaste and protect her from devilish acts.
If the frigidity is not complete, i.e. the man can have sex with his wife but at a wide intervals and this does not satisfy his wife's sexual desires, in this case, it is also lawful for the woman to seek divorce because of the harm that befalls her. But this harm differs from one woman to another woman because of the different nature of people in this respect.
We advise this sister and her husband to seek the help of doctors and maybe some drugs could help solve this matter and avoid separation which is not good.
Allah knows best.