Visitation rules at juvenile facilities are significantly more restrictive than at county jails, even when the person being held is technically an adult. Juvenile halls typically limit visits to immediate family members only, meaning parents and sometimes siblings. Friends, significant others, and extended family are generally not permitted regardless of the inmate's age.
The reasoning is that juvenile facilities are designed to protect and rehabilitate young people, and controlling outside contacts is part of that environment. An adult housed there for classification or overflow purposes is still subject to the same rules governing the facility.
Before you make a trip, call OC Juvenile Hall directly and confirm their current visitation policy, who is on the approved list for your friend specifically, and what the visiting schedule looks like. Do not assume the rules from the county jail carry over, because they typically do not. Getting that information ahead of time saves a wasted trip.