There is nothing that links a client directly to a media server.
Clients are assigned to Policies, and Policies are assigned to a Storage Unit or Storage Unit Group.
A Storage Unit is linked to a media server.
So, you need to check all the Policies where the client name exist, then check Attributes and Schedules for Storage Unit (or Storage Unit Group) name.
If the media server name is not clear from the STU name, you need to check the STU Properties.
I have move this post to NetBackup forum as this query is not related to NBU Appliances.