Legal Question in Traffic Law in California

There are several layers to this analysis.

First - was the stop valid? Did they have reason to pull him over in the first place? If they found something, his attorney may want to challenge the entire stop. Ordinarily, the police must justify a stop by pointing out a vehicle code section they allege the person was in violation of. I'm not sure what crime (even a traffic infraction) they will claim, but the entire stop may be challenged.

From there, a standard term of probation is that they must disclose their probation and search terms to police if contacted. If that's the case, then once contacted, the police were authorized under the search waiver to conduct a search.

The standard language (at least here in Orange County) is that a person under a search waiver can be searched "any time of the day or night by any peace or probation officer with our without a warrant, reasonable suspicion or probable cause." It's a broad umbrella that covers virtually every search unless it can be shown that it was done purely for harassment. Ordinarily, that's only alleged if the probation officer is searching every day at all hours and it's clear they're just out to annoy the probationer. Those are rare. The scenario you present is how many probation searches are conducted.

You never mention that they found anything, so this may all be a moot point. If they found something, he'll need an attorney to challenge the original stop. If they didn't find anything, then there's nothing to suppress and there really is no remedy for what may well be a valid search.