you can define this access rule in which ever entity is the owner of the association. I think in this case its the table "Employee", and you need to define an access rule for the module role user.
See the documentation here: https://docs.mendix.com/refguide/security
Access to Entity attributes and associations is controlled via Access Rules which you can configure at Entity Level. Go to the domain model, open the properties dialog on the Employee entity (double click) and select the Access Rules Tab. There you can specify the access that module roles have to the Entity Attributes and Associations. Note that Security needs to be activated before you can so this (see Project Settings -> Security)
You could start out with the Mendix online tutorials to learn more about Mendix and how it works:
The specific Mendix tutorial on module access:
I would suggest following these courses first, which will solve lots of your questions.