I would ask the client. However one caveat: unless the employer is in law enforcement or certain government jobs that involve the military or other sensitive jobs, an employer cannot ask about or use arrests as a basis for an employment decision. Convictions yes. Arrests only, no.
Good luck to you.
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I would proceed with a great deal of caution in this area without specific legal advice. Given that you are asking this question on a public forum, I would assume that neither you nor your client have an employment attorney to guide you here. That is a mistake. Background checks involving criminal records are highly regulated, and there are a number of specific requirements regarding this type of "investigative consumer report," as it is officially called. The California Investigative Consumer Reporting Agencies Act (Civil Code 1786 et seq.) and the Fair Credit Reporting Act (15 U.S.C. 1681a) contain some of these laws, but their implementation is tricky and require that the potential employee provide a knowing authorization to have this information searched and an opportunity to object to any adverse findings within a reasonable time, among other requirements. If an employment decision is made based on the results of a background check that was not properly conducted, that is a lawsuit waiting to happen.
I don't mean to scare you, but please be careful.
Disclaimer: This reply does not constitute legal advice or the establishment of an attorney-client relationship, and constitutes only general guidance based on the limited information provided, and may not take into account additional relevant facts and circumstances pertaining to your specific situation.
The employees are yours. So you need to comply with the state and federal Fair Credit Reporting Acts. If you don't, you could incur liability. Also, if you give clients too much information, and they wrongfully discriminate, you could be on the hook as well.
I am not only a lawyer, but I own a company that performs background checks. Most background check companies do it wrong and jeopardize their clients. You need to get a background check company that won't jeopardize you or your client.