Sub contractor client has no CIS paperwork

A new client came to see me yesterday.

He worked for a contractor for 17 weeks last year.

He raised invoices to the contractor.

The contractor issued paperwork which shows full amount of invoice less 20% tax, and my client was paid the net amount.

There seems to be no formal CIS certificates.

My client should be due a tax refund.

What paperwork do I need to get the tax refund for him?