At USP Big Sandy, which is a high-security federal penitentiary in Kentucky, segregation is called the SHU, the Special Housing Unit. It is solitary confinement, and inmates end up there for one of two broad reasons: disciplinary or administrative.
The disciplinary side covers a wide range of infractions. Fighting is the most common reason at a high-security facility like Big Sandy, but the list also includes possession of contraband such as a cellphone, drugs, or tobacco, failing a drug or alcohol test, threatening another inmate or staff member, gambling, stealing, and in serious cases an escape attempt or suspicion of one. Any of those can land someone in the SHU pending a disciplinary hearing.
The administrative side is less about punishment and more about investigation or protection. If your friend is suspected of involvement in something the facility is looking into, they may have been placed in the SHU while that investigation plays out. Alternatively, if there is a credible threat against him, administrative segregation can be used to protect him from other inmates while the situation gets sorted out.
Communication from the SHU is severely restricted. Phone calls are limited, mail moves slowly, and visits may be suspended depending on the reason for placement. Sending a letter through InmateAid is one of the more reliable ways to reach someone in that status. Keep it simple and supportive, and let him know you are there when he is able to respond.