Medi-Cal confusion

hello, looking for an answer to:

If a patient has Medicare and Medi-Cal as the secondary and she sees a provider that is a Medicare participant but NOT a Medi-Cal participant, can the provider bill the patient for the remaining 20% that Medi-Cal would pay if the provider was a participant? or is that against the law in California?

Thank you in advance.

Medi-Cal confusion