You can view the images that your client has hidden in a gallery by doing the following:
From the main menu click on Photos » Galleries.
Locate your client's gallery and click on Photos » Manage Categories.
Click on the name of a category to view the photos within it.
Any image that's been hidden by your client will have an icon of an eye with a slash through it displayed with it.
Note: You aren't currently able to un-hide images from the admin area.