Legal Question in Criminal Law in California

There are many exceptions to the warrant requirement. It's quite possible that one or more of them applied here. For example, police may conduct a search if they have permission from someone authorized to grant it, or even someone whom they reasonably but mistakenly believe had that authority. They also don't need a warrant when they are in hot pursuit of a fleeing felon and when the search is reasonably necessary for that pursuit. There are other exceptions as well.

You ask "What is going to happen in court and can he fight it," but it's not clear whether you mean your friend or his brother. It's also not clear whether this would be a parole violation case, a new criminal case, or both. Anyone the evidence might be used against can fight it, but I don't have enough information to say what arguments he could use or how strong they would be.

If your friend was not on probation or parole or subject to search for any reason, then there may be a good defense. The warrant will specify what they are looking for. If it's a search warrant to look for a person, then looking in a place that a person could not possibly be hiding exceeds the scope of the warrant. But if the warrant allows them to search for paperwork or something else small, it may have been discovered during a legitimate search.

Your friend is going to need a good attorney who is good with search issues.