Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
A person who conditions the divorce of his wife on the occurrence or taking place of a given matter, then that matter happens that divorce is valid according to the majority of the Fuqaha (Islamic Jurisprudents), including the four schools of thought.
Therefore, the speaking of this lady to her sister puts the bond of her marriage into danger. So, she should not speak to her. However, some scholars say that whoever intends by his oath just to prevent his wife from a given matter or to urge her to perform a given matter, and did not intend to divorce her, then the matter on which he conditioned the divorce happens, he has to expiate his oath and his wife is not divorced. This is the opinion which is considered the most preponderant according to Sheikh al-Islam and those who are of his opinion.
However, we advise the person who is in this situation to present his issue to the Islamic courts in his country.
Allah knows best.