Parental Alienation Syndrome has not been "recognized" in any published case to date. However, the damage to children resulting from parental alienation is well known to Idaho judges, including the justices of the Idaho Supreme Court. There is no category for it in the list of "best interests" factors, but it is relevant to more than one listed factor. In Doe v. Doe, 149 Idaho 669 (2010), the alienation was proven by audiotapes of telephone and personal conversations collected by the father. In that case the modification from joint legal/primary physical in mother to sole legal/primary physical in father was upheld by the Supreme Court. In oral argument it became clear that the justices were very concerned about the recorded alienation.
In order to set up the modification for maximum chances of success, you would be wise to work with an experienced family law practitioner to build your evidence, which might include professional interviewing of the children when they return from visits. The ages of the children, which you don't mention, could be significant in efforts to document the inappropriate comments.
Best wishes for a favorable outcome, and please remember to designate a best answer.
This answer is offered as a public service for general information only and may not be relied upon as legal advice.
Sign up to receive a 5-part series of useful information and advice about child custody law.