Is it permissible for the family of a deceased person who did not pray to pray for him at every obligatory prayer twice; once for themselves and once for the deceased?

If someone who abandoned prayer died, and they were denying its obligation, then they are an apostate disbeliever by consensus. However, if they abandoned it out of laziness:

They do not disbelieve according to Abu Hanifa, Malik, and Al-Shafi'i, but they are imprisoned until they pray (Abu Hanifa) or executed as a Hadd punishment unless they pray (Malik and Al-Shafi'i). The well-known position of Ahmad's Madhhab is that they disbelieve and are executed for apostasy.

It is not permissible to make up prayers on behalf of the deceased.