A warrant does not disappear when someone turns 18, and a no-bill warrant, which typically means the grand jury declined to issue an indictment on the original charges, is worth understanding clearly before your daughter takes any action. A no bill from a grand jury means the charges were not formally advanced at that time, but it does not necessarily mean the case is completely closed or that the warrant has been lifted. Prosecutors can sometimes refile depending on the circumstances.
The age transition from minor to adult changes which court has jurisdiction going forward. Any matter handled now would be in adult court rather than juvenile court, which is a significant difference in terms of consequences and record.
The most important thing your daughter can do is consult with a criminal defense attorney who knows Ventura County before she does anything else. An attorney familiar with the local courts and prosecutors can look at the warrant, determine exactly what it says, find out whether the no-bill effectively ended the matter, and advise her on the safest path forward.
If there is still an active warrant, voluntarily turning herself in with an attorney present, rather than being picked up unexpectedly, puts her in a much stronger position. Walking in proactively with legal representation signals good faith and typically produces a better outcome than waiting to be arrested. The sooner this is resolved, the sooner she can move forward without it hanging over her.