You may find the information at the link below helpful:
My initial thought is that really it should all be going through your clients invoice as zero rated materials provided and potentially no one should be claiming back the VAT as this is 'blocked'.
Do the materials go through the books or is it actual cash that is exchanged? If so, then it sounds like your client is just picking up the materials on behalf of the contractor and in this case he should pass over the receipt to the contractor.
If your client is using his own money, then potentially a VAT claim exists and a zero rated recharge for materials should be made to the contractor and CIS calculated just on the labour.
Not knowing the full circumstances it's difficult to provide any further guidance.
Hope the above does help.