Does divorce take effect or is expiation obligatory for one who unintentionally swore an oath by divorce concerning something that then occurred, which was his friend remaining after prayer and their then leaving together?

As long as there was no breach of the oath, you are not required to offer expiation, nor is a divorce counted against you. We alert you that the permissible oath is by Allah Almighty. As for swearing by divorce, it is among the oaths of the wicked, and it may lead to undesirable consequences.