I had a look in the notes of PG's book and sure enough, she says:
"The conspiracy to accuse her of witchcraft, or treason, as an alternative to divorce is well documented, especially by the historian Retha Warnicke, and was clearly as much of a lie as other evidence about her marriage given to the inquiry."
I haven't read Warnicke's book on Anne of Cleves yet so I don't know whether Warnicke really says that Anne was going to be charged with witchcraft. I know that Walter, Lord Hungerford, was accused of conspiring with witches and that it was believed that witchcraft could cause impotency, something that Henry VIII suffered in his relationship with Anne, but that is very different to Anne being linked to witches. I'm going to ask some historian friends to see if they have read Warnicke's book, but I don't believe that Henry VIII was ever going to frame Anne for witchcraft.
Update: A friend of mine who has read Warnicke's book recently said " I literally just finished Retha's book on Anne of Cleves yesterday! She says that Henry thought it possible that witchcraft was making him impotent and that is why he went after Hungerford and Cromwell. Cromwell being linked to Hungerford's witchcraft and buggery apparently. Retha never states that Anne was practicing witchcraft in any form or way nor does it ever cross Henry's mind that she is. To him, she was simply contracted to the Duke of Lorraine and because he knew that, he was unable to perform his "marital duties". Phillipa Gregory is utterly twisting Warnicke's words and completely making things up as usual."
I hope that helps!