What you are describing is called a separatee designation. This is a classification the Bureau of Prisons and state prison systems use to manage inmates who have a known prior relationship with someone at a facility, whether that is a staff member, a co-defendant, or another inmate. The concern is that the relationship could compromise security, be exploited, or create an appearance of impropriety even if no actual wrongdoing has occurred.
Separatee placement falls under administrative detention rather than disciplinary segregation. That is an important distinction. Disciplinary segregation is punishment for a rule violation. Administrative detention is a management decision, meaning the inmate is not being punished in the traditional sense but is being housed in the SHU while the situation is assessed or while the facility determines how to manage the relationship. The conditions may be similar, but the legal and procedural basis is different.
Yes, it is frustrating, and to the inmate and their family, it can feel wildly disproportionate. But from the institution's perspective, a pre-existing relationship between an inmate and a staff member is exactly the kind of dynamic they are trained to flag and isolate. It does not require evidence of wrongdoing, only the relationship itself.
The inmate can request clarification on the basis for the administrative detention through their counselor or case manager and ask for a review or a timeline for resolution. That is the proper channel.