Getting the full story from the outside is genuinely difficult, and your suspicion that you are not getting the complete picture is not unreasonable. Inmates often soften or omit details about disciplinary situations to avoid worry, judgment, or difficult conversations with the people they love.
There are two distinct reasons someone ends up in the hole, and understanding which one applies changes everything.
Administrative Segregation is not necessarily disciplinary. It can be used when there is a credible threat to the inmate's safety, when an investigation is underway, or when the facility needs to separate someone from general population while sorting out an incident they may or may not have been directly involved in. An inmate can land in administrative seg without having done anything wrong, simply because they were nearby when something happened or because someone made an accusation that has not been verified yet. In that case there is no final disposition until the investigation concludes.
Disciplinary Segregation is a punishment for a rule violation that has been adjudicated through the facility's hearing process. That means something was proven or admitted to before the placement happened.
As for finding out the actual reason from the outside, your options are limited. The facility is not obligated to share disciplinary details with a girlfriend. Your best avenue is calling the facility and asking to speak with his case manager or counselor, explaining that you are trying to understand his housing status so you can support him appropriately. A calm and cooperative tone sometimes yields more information than the official policy technically requires.
The most direct route remains asking him directly and pressing for specifics rather than accepting a vague answer.