The best course of action for the auditor in this scenario is to disclose the potential impairment to the customer before accepting the consulting engagement. By doing so, the auditor maintains transparency regarding any conflicts of interest and allows the customer to make an informed decision about the auditor's objectivity.
Disclosing prior involvement ensures that both the auditor and the client acknowledge and address any potential bias that could affect the outcomes of the consulting service.References: IIA's International Standards for the Professional Practice of Internal Auditing and Code of Ethics.