Is a woman required to offer a specific expiation for engaging in anal intercourse, especially since her husband has died and she has repented?

There is no expiation (kaffarah) for either the husband or the wife for engaging in anal intercourse. However, disciplinary action (ta'zir) is obligatory for committing a sin for which there is neither a prescribed punishment (hadd) nor an expiation. This sin is atoned for by sincere repentance (tawbah nasuh).