Legal Question in Business Law in California

The client most certainly can sue the consultant for any breach of the consulting contract, regardless of whether the breach was directly by the consultant or by a sub-contractor. In that regard it's not materially different from a situation in which and employee of the consultant commits the breach. The consultant is liable for fulfillment of his or her contract, no matter who actually does the work.

As for whether the client can sue the subcontractor directly, that is far less clear. The client generally could not sue the subcontractor due to lack of privity of contract, but in some cases if the subcontract is a true third-party beneficiary contract then the client could sue to compel performance. There are also circumstances in which the subcontractor's misconduct would give rise to some sort of cause of action other than breach of contract that might give the client a direct right of action against the subcontractor. It would all depend very much on the documents, details and circumstances of the particular situation.