What is the ruling on someone who swore not to eat or drink in a person's house, then forgot and ate and drank? And after performing the expiation, is it permissible for him to eat and drink in that person's house?

If you took an oath not to eat at your friend's house and you forgot, then no expiation is due from you.

However, if your friend is righteous, it is better to expiate for your oath and eat at his house. Otherwise, do not eat at his house and do not visit him, especially if he openly commits sins.

The expiation is by feeding ten needy people or clothing them, based on the saying of the Prophet, peace and blessings be upon him: "If you take an oath, then see something better than it, expiate for your oath and do that which is better."