A JW sister, let's call her Marjorie, has a non JW father who died. She arranges for an elder to take the funeral at a crematorium
That is the root of the problem. Why is she arranging a JW funeral for her non-JW father? It is presumptious and disrepectful of her to do that. It is disrepectful to her father, it is disrespectful to her husband, and it is disrespectful to the grandson.
The funeral home director knows how to arrange a service and who could officiate in a manner consistent with the beliefs of the deceased (or lack thereof). That is what they do for a living!
The elder taking the funeral tells Marjorie that she needs to tell her son he is not welcome at the reception, otherise none of the JWs can attend.
At this point it would be appropriate to tell the arrogant hypocrite that his presence is disturbing to family members in their time of grief, therefore he is not welcome!
The son goes anyway
Good for him! His grandfather's funeral is for the family's solace, not a command performance for the Board of Elders.