Legal Question in Personal Injury in California

The plaintiff's attorney is not allowed -- much less required -- to help the witness once there is a conflict between the witness and the plaintiff. (Depending upon the facts it might well be in the plaintiff's interest for his lawyer to help the defendant withdraw his statement. I presume the lawyer has decided that is not the case here.)

This does not mean the witness is stuck with his prior affidavit. It just means this one particular lawyer won't help him. He can get the help of another lawyer. Or he can act on his own behalf.

Whatever he does, he should bear in mind that he may be admitting he committed perjury, which is a serious crime. He should get his own lawyer to advise him about this risk, since the plaintiff's lawyer has a conflict of interest.

How the plaintiff's lawyer should deal with the plaintiff is another story. If he no longer believes his client is telling the truth, then he cannot let his client offer false testimony to the court. But your premise that the lawyer knows his client is lying may very well be false. The lawyer may still believe his client, either because he does not believe the witness or because the witness's testimony is relatively unimportant. That's especially true if there is other evidence that supports the plaintiff's story and contradicts the witness's.