Question: If a relative of a deceased person offers dua and istigfar or does good deeds as a isale sawab, can this dead person know where it is gifted from?

Answer: Yes, the deceased person will be informed by the angels by the decree of Allah s.w.t that your relative has done this virtuous deed, did sadaqah for you etc. And in lieu of these good deeds Allah s.w.t gives mountain amount of blessings towards the dead person.

Reference: [1] Al-Bayhaqi fee shuabil Iman under authority of Mishkat Al-Masabih p-206 Hadith:2246