The man she was having sex with was not the man she consented to have sex with.
She consented to have sex with him, that's it. No, it's not right, but it's definitely not rape either. Calling it rape is just wrong and does injustice to real rape victims.
More accurately, he was Mark Kennedy pretending to be someone called Mark Stone. Saying he was Mark Stone didn't magically make him Mark Stone.
>> She consented to have sex with him, that's it.
Because she didn't know he was an undercover policeman. He obtained her consent by abusing his authority as a police officer. His superiors also abused their authority by ordering or permitting this to happen. They took away her ability to make an informed decision about whether she wanted to enter into a sexual relationship with that man.