There is no Sahih Hadith that mentions the fazeelat of reciting the kalima tayyiba 70,000 times. However, it is said by Sheikh Muhiudinn Ibn Arabi in the book "Mirqaat Sharh Mishkat" that a narration from the Prophet (peace be upon him) reached him stating that whoever recites "la ilaha illallah" 70,000 times will be forgiven, and whoever it is recited for will also be forgiven.
Thus, no sahih hadith supports the fazeelat of the practice of reciting the kalimah tayyiba 70,000 times for the forgiveness of the deceased; however, it is one of the "experiences" of the religious elders. Therefore, if a person recites the kalima tayyiba many times without considering it obligatory or Sunnah, specifying a particular quantity, or associating a distinct fazeelat with it, then it will surely bring them reward. Similarly, if it is recited as a form of Eisal e Sawab for the deceased, then Insha'Allah it will be a cause for their forgiveness.