In the Admin under the users account, you will need to setup Account Limitations. When editing the account, you will see a button labeled Add Limitation about half way down the page. If you only want the user to see that particular interface and its associated node, you can setup a Single Interface Limitation and they will only be able to see that interface and the node that interface is on.
By default NPM is setup to show users information about lots of nodes. To make it easier for the user, you can setup customized views and assign them to that user. Views are also managed through the Admin. Once you have a view created, you can assign it to the user by editing the Views section under their account.