ADP has no legal responsibility to the employee. ADP's contract is with the company, not the employee; the employee works for the employer, not for ADP. As a payroll manager for many years for several very large employers, here's what has to happen.
1. Employee files police report.
2. Employer send police report to ADP (since the checks were cut on ADP's account, from what I understood from the OP)
3. ADP files an affidavit of forgery with their bank.
4. Their bank investigates and, at some point in the future, the bank will release the funds back to ADP.
5. ADP will refund the monies to the employer's bank account.
Now, the employer can choose to front the money to the employee and, depending on the reliability of the employee, I have done that in the past, with a properly executed repayment or deduction agreement.
Here's the problem you, as the employer, may have. If you can't show that the employee actually got the check (for example, you don't have employees sign for the release of their paychecks), and the employee files a claim for unpaid wages, you would probably be ordered to pay. Of course, by that time I can pretty much guarantee you that the whole process as described above will have been completed; wage claims, especially in California, do not get resolved overnight.
Horse out of the barn now, but best practice is to have employees sign for their paychecks. Even better practice is to really push direct deposit and I'll bet the employee will see the advantage of direct deposit now.