If it is a covered benefit, and you took the patient KNOWING they were Medi-cal AND you are not participating in Medi-Cal at the time you provided services, you CAN NOT bill the patient.
He CAN bill the patient's who received services once he became a participating provider. The rest is a write off, sorry. The only good thing is that the write off shouldn't be too much as Medi-Cal has HORRIBLE reimbursement.