Does informing the person on whom the expiatory charity (kaffarah) was spent diminish its reward, even if the purpose of informing him is to bring joy to him?

It is permissible to give charity from one's wealth to a poor person or an orphan, then to gift the reward of that to a living person, or to include them in the reward. Secret charity is better than open charity because it is closer to sincerity and further from ostentation, unless there is an overriding benefit in making it public, such as people following suit, in which case open charity is better. If informing the colleague brings her joy or encourages her to act, then there is no harm in informing her. If the sponsorship is recurring, it is preferable to inform her so that she may have the intention of sponsorship.