This.
It should be a very quick trial and immediate execution. That would be justice for her.
If she's a citizen she's admitted to committing treason by fighting/giving comfort to the enemy that we are at war with, which carries the death penalty. So if she wants to come back, we kill her. She really should think twice about this if she wants to live.
But how do you feel about about adherence to Russia, giving them aid and comfort within the United States by easing sanctions?
https://www.law.cornell.edu/uscode/text/18/2381
Whoever, owing allegiance to the United States, levies war against them or adheres to their enemies, giving them aid and comfort within the United States or elsewhere, is guilty of treason and shall suffer death, or shall be imprisoned not less than five years and fined under this title but not less than $10,000; and shall be incapable of holding anyoffice under the United States.
(June 25, 1948, ch. 645, 62 Stat. 807; Pub. L. 103–322, title XXXIII, § 330016(2)(J), Sept. 13, 1994, 108 Stat. 2148.)