Is expiation obligatory for each time a friend was not informed of the sin, or is it sufficient to inform him of all the times the sin was committed to fulfill the oath?

The answer is summarized as follows: The legitimate means of restraining oneself from sin is to strive against it, to be mindful of Allah, and to recall the ominous consequences of sins. Ibn al-Qayyim said: "No blessing is removed from a servant except due to a sin, and no misfortune befalls him except due to a sin."

Frequent swearing is reprehensible in Islamic law. A Muslim is commanded to conceal his own faults, and it is disliked for him to disclose his sins.

If an oath is taken to perform something disliked, it is recommended to break the oath, and it is disliked to fulfill it.

As for the phrase "كلما" (every time/whenever), it does not imply immediacy unless intended or indicated by a context. So, if you did not intend immediacy in your oath, then delaying informing your friend about what you did is not considered a breach of the oath. Informing him about the times you committed the sin is considered fulfilling the oath, and no expiation is required of you. However, if you intended immediacy, then delaying informing him is considered a breach. If you broke your oath and did not offer expiation for it until you broke another oath, then only one expiation is required of you.