ANSWER:

In sha Allah you did not but you should have brought it to his attention that this is an innovation and the prophet salla Allahu alaihi wa sallam had never done that in his life. 

QUESTION: I was driving my grandfather and we were passing a graveyard. He said: “Stop here so I can recite Al-Fatiha to someone in the graveyard.” I knew that reciting Al-Fatiha is an innovation but I still stopped so he could visit the graveyard but I hoped he would also do something beneficial for the deceased like making a dua. Did I assist the sin?