My mother went to hajj and she always prays she never missed a Fard and always reads the Qur'an, but her problem is that she look in the coffee cup and read it as she knows about (al-Ghaib) she always does that and I always argue with her what is she in Islam law and should I obey her?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Know that foretelling events (or fortune) by the positions and aspects of stars, by looking in the coffee cup or examining the lines on one's palm is completely Haram in Islam. Such an action is, in fact, one of evil deeds that Allah and His Messenger have forbidden to all. Such an action is based on illusion and delusion. Thus, many Prophetic Hadith warned us against deeds mentioned above and stated punishment for those who practise them.
Ibn 'Abbas (Radiya Allahu Anhu) reported that the Prophet (Sallallahu Alaihi wa Sallam) said: "Whoever learns something about stars has, in fact, acquired knowledge of magic, and the more knowledge he receives (in stars) the more will be in magic" . [ Abu Dawood with a sound chain of narrators].
And in the Sunan of al-Nasa'i from the Hadith of Abu Hurairah "Whoever depends and puts his trust (instead of Allah) in something, Allah will leave him to what he trusts in" . In other words, Allah does not only leave people who deal with fortune-tellers and augurs but also deprives them of His help and success.
It is recorded in Sahih Muslim that the Prophet (Sallallahu Alaihi wa Sallam) said: "Whoever goes to a fortune teller to ask him about something, Allah will not accept from him the prayers for forty days" . The previous punishment is just for asking a question, but if such a man believes the saying of a fortune-teller, then the punishment will be as stated. In a Hadith reported by Abu Dawood , al-Tirmizi , Ibn Majah and al-Nasa'i : "Whoever goes to an augur and believes in what he tells him, he becomes an unbeliever in what was revealed to Muhammad (Sallallahu Alaihi wa Sallam) (The Mission of Islam)" . Because Allah Says (interpretation of meaning): {Say: "None in the heavens and the earth knows the Ghaib (Unseen) except Allah, …} [27: 65].
Another Hadith read: "Whoever practises omens or omen is done for him is not of us; whoever augurs or augury is done for him is not of us" .
Thus, a Muslim should avoid such non-Islamic practices and should warn Muslims (and people in general) not to seek help of augurs, since fighting such acts constitutes a true belief in Allah and His Messenger as well as a manifestation of guarding true creed.
Allah knows best.