Legal Question in Criminal Law in California

If he plead guilty at arraignment (I have no idea why somebody would do that without even looking over the police reports to see if the DA could even prove the case in the first time), the he would likely have been told what he would get if he plead guilty. If he plead "open", meaning he could get anything from no more jail time up to the maximum, then he's just gambled in a huge way and has agreed to accept whatever punishment the judge wants to impose.

Whoever "he" is should be talking to a defense attorney about this case ASAP. If he wants to try and withdraw his plea and fight the case or at least verify that the case is solid, it MUST happen before sentencing or it's too late.

Something is missing from your question - either you just don't know what the sentence promised is or "he" has potentially made a huge mistake.