It is her own words that trip her up . On the night - she "knew" he had done it.
And even when he was planning there was a possibility she was not sure he would carry it out - and then in another sentence she said she did know he meant it - contradiction.
But she had the opportunity when with the police on her own at the mortuary to tell them everything and get him caught immediately - and because she did not - and the fact that the moderator might not have been found makes it obvious that she DID pervert the course of justice. If he is guilty then he nearly got away with it because of her.
There is no legal duty to tell everything you know. You can't make up a lie like claiming the person was with you when they were not. You can't help get rid of physical evidence.
If he told her Nevill never called him but he wants her to tell police he called her to say he was worried because Nevill called him then she is actively help obstruct justice. She was very careful in what she said about such to avoid presenting such a scenario. that is where she had the greatest chance of being prosecuted. Obviously Jeremy didn't contradict her and say he told her he received no call but asked her to lie and pretend he told her he had. His claim is the only thing that could have established such but he would have to have confessed to do that.