How to prove self employment

Posting on behalf of a friend, not sure if the right platform to post on. They're currently in the process of being vetted for a disability assessor role. They've provided their working history which also includes being self employed. The self employment has not earnt them over the £1000 so they have therefore not completed a return. The vetting company are asking for "proof of self employment" and have asked for copies of client information as proof. My friend does not want to provide this due to GDPR. What other "proof of self employment" can they provide that would be sufficient for the vetting company?

Hi,
They would need to ask the vetting company what other evidence they would accept.
Thank you.