What is the expiation for someone who swore by divorce not to go with a friend to a certain place, then went with him to that same place on the same day?

The fatwa that we adhere to is that swearing by divorce effects a divorce upon breaking the oath, and this is the view of the majority of scholars. So long as you have broken your oath, a divorce has been effected upon your wife. If you have not divorced her more than once before this, you may take her back before her waiting period (iddah) expires. The legitimate oath is by Allah Almighty, whereas swearing by divorce is among the oaths of transgressors.