She used to take money from her father without his knowledge, and he does not pay her mobile phone bill. Should she return the money?

It is not permissible for the child to take money from his father without his knowledge, unless the father is falling short in obligatory spending, in which case the child may take as much as will suffice him, on a reasonable basis. That is because of the report narrated by al-Bukhari (5364) and Muslim (1714) from ‘Aa’ishah (may Allah be pleased with her), according to which Hind bint ‘Utbah (may Allah be pleased with her) said: O Messenger of Allah, Abu Sufyan is a stingy man, and he does not give me enough to suffice me and my child, except what I take from him without his knowledge. He said: “Take enough to suffice you and your child on a reasonable basis.”

Anything beyond maintenance remains prohibited, because it is not permissible to take the wealth of a Muslim except with his willing concept.

Therefore you must return what you took from your father. If you do not know how much it was, then return whatever you think is most likely to be the correct amount, so that you will absolve yourself of any responsibility. You do not have to tell your father about that; rather it is sufficient to make sure that the money reaches him by whatever means, even if it means giving small amounts until the debt of paid off.

The Permanent Committee for Ifta’ was asked: Since I was small, I would see my father putting something, whether it was cash or any other useful thing, and I would take some, and my father was not aware of it. After I grew up, I began to fear Allah and stopped doing this. Now is it permissible for me to admit to my father what I did, or not?

They replied: You must return what you took from your father of cash and other things, unless it was something small for the purpose of maintenance, in which case there is no blame on you."(Fatawa al-Lajnah ad-Da’imah  15/352).

Your father is not obliged to pay your mobile phone bill, unless the mobile phone is used for necessary household matters and the like. If that is the case, and your father refuses to pay the bill, it is permissible for you to pay what you owe from the money.

And Allah knows best.