I have also worked for one of the world's largest law firms, and I currently work for what is arguably the most prestigious university in the US and one of the top ones in the world. In HR. You know, that department that is responsible for maintaining employment laws.
There is no law in CA or anywhere else in the US that says managers cannot talk about their employees or former employees, with very limited exceptions which do not apply here. Many employers have adopted that as a policy. Mine has. Some may even believe it is law.
They are wrong if they do.