How can a husband whose wife has died (and he's become her non-mehram), pray for his wife for 'isal-e-sawab'?

In regard to your answer to Que.4938,someone asked me that there is a hadith that says that 'a husband/wife is a sadqa-e-jaariyaa for his/her deceased spouse'. So, how is it possible when a wife has died and her husband has become her non-mehram, that it is allowed for him to pray for and give 'isaal-e-sawab' to a non-mehram woman? And why is he allowed to look at her face (of her dead-body) when she's a non-mehram? A non-mehram male is not allowed to see a female's face, so why is it allowed in this case?