The journalist has (probably) committed libel, which is (mostly) not a crime in the US but can result in civil liability (i.e. B could sue the journalist). Since A and the journalist conspired to produce this fake news, they would likely share in this liability (assuming B can prove they acted in concert).
To establish a case of libel or defamation in the US, B will generally need to prove all of the following factors:
- The journalist caused something to be published.
- The material included a provable statement of fact.
- The statement was materially false. "Material" means that you can't sue someone for getting a minor detail wrong if the "gist or sting" of the overall publication is substantially true.
- The falsehood caused some form of cognizable damage to the plaintiff's reputation (i.e. it must be more specific and measurable than "he made me look bad" or similar arguments). Certain statements, such as allegations of criminal acts, are assumed to cause such damage ("defamation per se").
- The journalist acted with actual malice, meaning the journalist either knew the material was false, or made no serious effort to fact check it.
Actual malice is usually the sticking point in cases like this. If A and the journalist were sufficiently careful, there may be no evidence of it. Even if evidence does exist, it may be extremely difficult to establish to the satisfaction of a court.
(As I alluded to above, criminal libel is technically still a thing in some states. But it generally requires the state to establish at least all of the elements described above. In practice, prosecutions are extremely unusual.)
Regardless, it is highly unlikely that this would result in overturning an election which has already taken place, whether for the President of the United States, or for any other elected office whether state or federal.