How to customize a role to give access to a specific custom folder in reports and analytics

Requirement: Client wants to give access to a third party vendor to access custom reports in a specific custom folder in reports and analytics region. When I create a custom role and add the BI admin role to it, the third party user has access access to R&A, however, he is able to view all the reports. Is there any way to give access to a specific folder to the user?

Would highly appreciate your help!