Praise be to Allah.
It is not a condition of divorce that the husband should speak the word in front of his wife or that she should know of it. When the man speaks the words of divorce, or writes them down, this is regarded as a valid divorce that takes effect, even if the wife does not know of it.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) was asked:
A man has been away from his wife for a long time, and he had divorced her to himself, even though he did not inform her of that. Has divorce taken place?
The divorce has taken place, even if he did not convey that to his wife. If a man utters the words of divorce and says, “I divorce my wife”, the wife is divorced whether she knows it or not. Hence if we assume that the wife does not come to know of this divorce until after she has had three menstrual cycles, then her ‘iddah is complete even if she did not know. Similarly if a man dies and his wife does not come to know of his death until after the end of the ‘iddah period, then she does not have to observe ‘iddah because in this case it is already over.
Fatawa Ibn ‘Uthaymeen, 2/804.