Q: In XenApp 7.9 environment - Customer was unable to see the applications published to him.

A: The User did limited visibility to correct employees, however in the delivery group properties he did not assign anyone. Hence the apps were not visible, as they were not assigned to any one.
To better understand this:
In the delivery group we can provide access to any user/groups.
In the "limit visibility option" we need to further limit the visibility to a subset of the usernames provided in delivery group properties.
OR
In the "limit visibility option" we can show the apps to all the users in delivery group.
Summary:The delivery group assignment is always greater than the internal application property's assignments.
Internal app property assignment is a subset of delivery group's user assignment.
All the users must be a part of delivery group whoever we are assigning in property of application.