He can’t pardon her before she’s convicted of a crime
In 1866, the Supreme Court ruled in Ex parte Garland that the pardon power “extends to every offence known to the law, and may be exercised at any time after its commission, either before legal proceedings are taken, or during their pendency, or after conviction and judgment.”
Bingo. He can’t pardon for state charges or impeachment. He can pardon any time after the act or acts have been committed even if not yet prosecuted.