ANSWER:

It is not permissible to fast for deceased people unless they had an obligatory fast that they didn’t make up which you know for sure. 

Can we fast for the deceased when we are not sure if they had missed fasts?

Fasting, Funerals | November 6, 2018 7:40 am
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My question to you is that can one keep nafil fast (voluntary) and kadha fast (compulsory) together? If yes, then is it acceptable if one keeps it(both nafil and kadha fast together) without doing suhur?
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Can a husband wash a dead wife’s body and vice versa?