Child support with a step child

My wife has a son from a previous marriage who has lived with us primarily our whole marriage. Now that we are separated, her son will be moving back with his dad in TN and he is trying to redo the parenting agreement and she will probably go along with it. We have 2 kids together and she has agreed through separation agreement that I will have primary custody and she will have visitation every other weekend.
When using the child support calculator does she get a credit toward her for care of her son who does not live with her or does it still count for her as she is responsible for him even though she does not pay any support for him?

She does not get a credit for her son unless she pays child support for him, or unless he lives with her.