I'm pretty convinced that Marco B. was indeed selling drugs. But that's not the point. We have trials for a reason. Including the ability to effectively cross-examine the only witness to the crime.
Justice Rylaarsdam properly holds that the trial court here denied Marco that right when it prevented his counsel from discovering from the police officer where he was when he allegedly saw what he saw. Indeed, the Court of Appeal's opinion gives one a very bad impression of the trial judge -- Judge Deborah Chuang, in Orange County -- and (although Justice Rylaarsdam doesn't use this word) one might accurately describe the in camera hearing that she conducted as pretty much a cherade; or, more colloquially, a joke.
I understand that that police officer might not want to tell anyone where he was standing at the time; that way, s/he can use this location again, and potentially keep it secret. But the officer's preference in this regard has to give way to the right of a defendant to contest the charges against him, especially in a case like this. We can solve the problems about the "secret" location in other ways; e.g., protective orders, disclosure limitations, etc.
Justice in the Court of Appeal was a lot better here than it was in the trial court.