The Folder Gallery is not something that can be easily secured, but the Image Gallery Feature can be put on a private page based on roles and it enforces security. So you could create a role for the customer, then create a page and set the view roles and edit roles to that customer and put an Image Gallery on the page. If you don't want them to be able to use other features on the page then only add their role to page view permissions, then after you put the gallery on the page set the module edit roles to allow the customer role.
Hope it helps,
Joe