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.
It is obligatory for him to do so?
If he offers expiation for him, then he tells him, and he (the one who originally made the oath) accepts that and agrees to it, will that be sufficient?
The correct view is that that will be sufficient.
Is it conditional upon him telling the other person that he has offered expiation, so he will be free of blame?
Yes, he has to tell him, and if he tells him before he offers expiation, this is better. And Allaah knows best.