Who cares? You're not about to start suggesting that Ruth Paine was involved in the assassination conspiracy, are you?
Marina's estrangement with Ruth seemed to start right after the assassination and not years later when she became a conspiracist. She was isolated, afraid, dependent on others and both Robert and Marguerite, who she had to lean on for support, disliked the Paines intensely. I think they told simply told her not to trust the Paines, to stay away from them, they were using the assassination for their own interests and not hers.
Marguerite said this about Ruth: "The proud and perfect Quaker…I keep saying she is a fraud and liar…I can hardly read this woman’s thoughts. She is evil, and selfish and the cause of it all. You ought to be horse-whipped."
Robert particularly didn't like Michael either. He thought Michael lied about not knowing about the rifle in the garage. And he wrote in his book "Lee" that Ruth and Marina's friendship "apparently contributed to Lee’s feeling of rejection and failure."
I think it was a personality and cultural clash as well. Whether the assassination happened or not they were not going to get along. They came from two different worlds. Lots of wheels turning here.