This question is the result of a workshop held at the EACC meeting in March 2017
Can the CB offer internal auditing to its clients
March 2018
ISO/IEC 17021-1: 2015 – Clause 5.2.6
The consensus position of the EACC is that this is not possible for existing clients of the CB, internal auditing can be offered to other organisations not certified by the CB. There should be a suitable gap (2 years) between the CB offering internal audit and the customer becoming certified by that CB.

