It sounds like you have a security requirement that customers can only see their own data. Mendix has a built in way to do this using Entity Access Security. To accomplish this, you should create an entity called 'Customer' that is linked to all the important data and to the user's Account. Then use the Customer entity to configure entity access controls. That is, only allow a user to see data that is associated with the 'Customer' that the account is related to.
The best way to learn how to do this is check out the Mendix Learning Path for Advanced Security. I believe the Entity Access Security module would be helpful in this situation.