What is the ruling regarding someone who had intercourse with his former wife, who was observing the iddah of another man, during the daytime in Ramadan without desire?

The person committed several Shariah prohibitions: being in seclusion with a non-mahram woman (khalwa), embracing her, and doing so in Ramadan. He must repent to Allah for these actions. He does not owe expiation (kaffara) or make up that day (qada') unless something was discharged from him. It is recommended to follow repentance with good deeds, as Allah the Almighty says: "Indeed, good deeds do away with ill deeds."