How can I atone for the sin of my deceased husband, who was observant of religious duties, but committed the sin of knowing and speaking with a woman?

The question is incomplete. If the husband possesses the righteous attributes and deeds mentioned, we hope that these will be a cause for expiating his sins, in accordance with the Almighty's saying: "Indeed, good deeds do away with ill deeds." And the mercy of Allah is vast, and He may not hold him accountable for the evil deeds he committed. It is advisable to frequently supplicate for him, seek forgiveness for him, and perform righteous deeds.