Telling a person that his vow has been broken so he can offer expiation
Question

                If a person breaks the vow of another, without the person who made the vow knowing about that, does he have to tell him so that he can offer expiation?
Praise be to Allah.We put this question to Shaykh Muhammad ibn Saalih
    al-‘Uthaymeen, may Allaah preserve him, who replied:
He has to tell him.
Question:
It is obligatory for him to do so?
Answer:
Yes.
Question:
If he offers expiation for him, then he tells him, and he (the one who
    originally made the oath) accepts that and agrees to it, will that be sufficient?
Answer:
The correct view is that that will be sufficient.
Question:
Is it conditional upon him telling the other person that he has offered
    expiation, so he will be free of blame?
Answer:
Yes, he has to tell him, and if he tells him before he offers
    expiation, this is better. And Allaah knows best.