Firstly: 

It is permissible for a man to wash his wife, and for a woman to wash her husband, even if the woman is observing ‘iddah following a revocable talaaq (divorce), as stated previously in the answer to question no. 154381 

The fact that it is permissible for a man to wash his wife is indicated by the hadeeth of ‘Aa’ishah (may Allah be pleased with her), who said: The Messenger of Allah (blessings and peace of Allah be upon him) came back from a funeral in al-Baqee‘ and I had a headache and was saying, Oh my head. He said, “Rather, I should say, Oh my head. It will not matter if you die before me, for I will wash you and shroud you, then I will offer the funeral prayer for you and bury you.”

Narrated by Ahmad, 25380; Ibn Maajah, 1456; classed as saheeh by Shaykh al-Albaani in Saheeh Ibn Maajah, 1/247 

It was narrated from Asma’ bint ‘Umays (may Allah be pleased with her) that Faatimah (may Allah be pleased with her) left instructions that ‘Ali (may Allah be pleased with him) should wash her.

Narrated by ash-Shaafa‘i, 1/312; ad-Daaraqutni, 2/79; al-Nayhaqi, 3/396. Its isnaad was classed as hasan by ash-Shawkaani in Nayl al-Awtaar, 4/35 

Ash-Shawkaani said: The words of the Prophet (blessings and peace of Allah be upon him), “for I will wash you”, indicate that a wife may be washed by her husband if she dies.

End quote from Nayl al-Awtaar, 4/35 

As-San‘aani (may Allah have mercy on him) said concerning the hadeeth of Asma’ (may Allah be pleased with her): This indicates that this was something that was well-known during the life of the Prophet (blessings and peace of Allah be upon him).

End quote from Subul as-Salaam, 1/478 

This is also the view of the majority of scholars (may Allah have mercy on them). 

An-Nawawi (may Allah have mercy on him) said: With regard to washing his wife, it is permissible in our opinion and according to the majority of scholars. This was narrated by Ibn al-Mundhir from ‘Alqamah, Jaabir ibn Zayd, ‘Abd ar-Rahmaan ibn al-Aswad,… Maalik, al-Awzaa‘i, Ahmad and Ishaaq. It is also the view of ‘Ata’, Dawood and Ibn al-Mundhir. Abu Haneefah and ath-Thawri stated that he cannot wash her; this was also narrated from al-Awzaa‘i. They quoted as evidence the fact that the marriage has ended (with the death of one of the spouses), so it is akin to a complete and irrevocable divorce.

End quote from Sharh al-Muhadhdhab, 5/122 

In Majmoo‘ Fataawa ash-Shaykh Ibn Baaz (may Allah have mercy on him), 13/109, it says: 

Some of the fuqaha’ were of the view that the marital relationship comes to an end with death; what is your view on this issue? 

He replied: 

This is a view that is contrary to the Sunnah, so no attention should be paid to it. End quote. 

With regard to a woman washing her husband, this is supported by the hadeeth of ‘Aa’ishah (may Allah be pleased with her), who said: If I had known beforehand what I know now, no one would have washed the Messenger of Allah (blessings and peace of Allah be upon him) but his wives.

Narrated by Abu Dawood, 3141; classed as saheeh by Shaykh al-Albaani (may Allah have mercy on him) in Ahkaam al-Janaa’iz, 1/49 

An-Nawawi (may Allah have mercy on him) said: Ibn al-Mundhir narrated in his two books, al-Ishraaf and Kitaab al-Ijmaa‘, that the ummah is unanimously agreed that a woman may wash her husband. Others also narrated that there is consensus on this point.

End quote from Sharh al-Muhadhdhab, 5/114. 

And Allah knows best.