Praise be to Allah.
What appears to be the case from the husband’s words here is that he did indeed divorce his wife, and wanted to convey that divorce to her through her mother.
Based on that, his divorce of his wife did count as such, as soon as he uttered the word, even if his mother-in-law did not actually convey news of that divorce to his wife.
It says in al-Mabsoot by al-Sarkhasi (6/141): If he said to someone else: Tell my wife that she is divorced, then she is divorced, whether that person tells her or not. End quote.
In al-Mudawwanah (2/78), it says: What do you think, if a man says to another man: Tell my wife she is divorced – when does that divorce take place? Is it on the day that he tells her, or on the day the husband told him to tell her?
He said: According to the view of Maalik, the divorce occurred on the day the husband told him to tell her.
I said: What if he did not tell her?
He said: The divorce still counts as such, according to the view of Maalik, even if he [the second man] did not tell her, because Maalik said concerning a man who sent a messenger to his wife to tell her that he had divorced her, but the messenger withheld that news: That does not matter, because the divorce has already taken place. End quote.
See the answer to question no. 195336.
If the husband says to his wife: “[you are] divorced, divorced, divorced (taaliq, taaliq, taaliq)”, that only counts as one divorce.
The matter of a divorce issued in a state of anger is subject to further discussion. The view we follow on our website is that if the anger was such that the person did not know what he was saying, or he knew but his anger was extreme and pushed him to speak of divorce, and were it not for his anger he would not have issued a divorce, then the divorce does not count as such. Please see question no. 45174.
What you should do – if you live in a country where there are sharee‘ah courts – is refer to the court so that the judge (qaadi) may examine the case of this divorce.
And Allah knows best.