Why don't we just include the whole definition here for those who don't have a copy or don't feel like looking it up themselves:
So, independence is still required in the standard, and audits must be carried out by personnel not responsible for the object/activity being audited.
The answer to the OP's question about using another company's auditors? Yes it's allowed. As others have mentioned there are pros and cons to doing it.
ISO 9000:2015 said:
3.13.1
audit
systematic, independent and documented process (3.4.1) for obtaining objective evidence (3.8.3) and evaluating it objectively to determine the extent to which the audit criteria (3.13.7) are fulfilled
Note 1 to entry: The fundamental elements of an audit include the determination (3.11.1) of the conformity (3.6.11) of an object (3.6.1) according to a procedure (3.4.5) carried out by personnel not being responsible for the object audited.
Note 2 to entry: An audit can be an internal audit (first party), or an external audit (second party or third party), and it can be a combined audit (3.13.2) or a joint audit (3.13.3).
Note 3 to entry: Internal audits, sometimes called first-party audits, are conducted by, or on behalf of, the organization (3.2.1) itself for management (3.3.3) review (3.11.2) and other internal purposes, and can form the basis for an organization’s declaration of conformity. Independence can be demonstrated by the freedom from responsibility for the activity being audited.
Note 4 to entry: External audits include those generally called second and third-party audits. Second party audits are conducted by parties having an interest in the organization, such as customers (3.2.4), or by other persons on their behalf. Third-party audits are conducted by external, independent auditing organizations such as those providing certification/registration of conformity or governmental agencies.
Note 5 to entry: This constitutes one of the common terms and core definitions for ISO management system standards given in Annex SL of the Consolidated ISO Supplement to the ISO/IEC Directives, Part 1. The original definition and Notes to entry have been modified to remove effect of circularity between audit criteria and audit evidence term entries, and Notes 3 and 4 to entry have been added.
audit
systematic, independent and documented process (3.4.1) for obtaining objective evidence (3.8.3) and evaluating it objectively to determine the extent to which the audit criteria (3.13.7) are fulfilled
Note 1 to entry: The fundamental elements of an audit include the determination (3.11.1) of the conformity (3.6.11) of an object (3.6.1) according to a procedure (3.4.5) carried out by personnel not being responsible for the object audited.
Note 2 to entry: An audit can be an internal audit (first party), or an external audit (second party or third party), and it can be a combined audit (3.13.2) or a joint audit (3.13.3).
Note 3 to entry: Internal audits, sometimes called first-party audits, are conducted by, or on behalf of, the organization (3.2.1) itself for management (3.3.3) review (3.11.2) and other internal purposes, and can form the basis for an organization’s declaration of conformity. Independence can be demonstrated by the freedom from responsibility for the activity being audited.
Note 4 to entry: External audits include those generally called second and third-party audits. Second party audits are conducted by parties having an interest in the organization, such as customers (3.2.4), or by other persons on their behalf. Third-party audits are conducted by external, independent auditing organizations such as those providing certification/registration of conformity or governmental agencies.
Note 5 to entry: This constitutes one of the common terms and core definitions for ISO management system standards given in Annex SL of the Consolidated ISO Supplement to the ISO/IEC Directives, Part 1. The original definition and Notes to entry have been modified to remove effect of circularity between audit criteria and audit evidence term entries, and Notes 3 and 4 to entry have been added.
The answer to the OP's question about using another company's auditors? Yes it's allowed. As others have mentioned there are pros and cons to doing it.