It's a mess. The person who reported on the alleged Cunningham / Bartlestein affair seems to have just thrown it out there to stir shit up and make Bartlestein and Suns management look irresponsible and harmful. It is thrown out there without any real evidence, but people love a good scandal, so it has caught a lot of attention.
You could imagine Cunningham or Bartlestein suing the person for saying that about them. But my (non-lawyer) understanding of such things is that it is hard to win a defamation lawsuit. You have to be able to reasonably prove that they made a false statement, that it was harmful to you in some clear measurable way, and that they knew it was false when they wrote/made it. The person who made the statement can just say that they had heard the rumor and believed it was true at the time, and that they were just passing on what they heard. They probably would not be found liable.
Just a bad situation all around. Suns ownership and management look bad, and now people will slut-shame Sophie Cunningham. There are things I don't like about Cunningham, but I also dislike how sex scandals seem to disproportionately cast shame on the woman. The man is often seen as a stud for having sex with the woman, and the woman is seen as a slut for being promiscuous with the man. I don't think Cunningham deserves that and I hope that isn't the way this sticks for her.
Don't know about the US, but here in Belgium and France, that's definitely a lawsuit with prison. Because it's on social media (that, alone, is enough), and because there's public shaming and potential professional consequences.
Lucky.... I wish our defamation and libel laws were stronger.