A safer way to allow one specific client to access a backup image catalogued under one other specific client name is to configure the Distributed Application Restore Mapping on the master server.
Other way to browse other client backup images is, link the image folder under /usr/openv/netbackup/db/images/. Then you will be able to browse the images of client B from client A. Not sure this is the right way to do it. Typically this is done for Oracle RAC servers.
As per excellent explanation by @Amol_Nair, client restore for itself is enabled by default.
You can see the the global setting for this in Host Properties - Master server - Client Attributes.
See this extract from NBU Admin Guide:
See Allow client browse and Allow client restore.