The secondary user must set up their own Arlo account to view shared devices. When sending an invitation, the invitation email will contain a link to set up a new Arlo account. Once the new Arlo account is set up using the link in the invitation, the device will be shared.
For more information on how this is done, take a look here: How do I add friends to my Arlo account?
If the account is already created, she will need to click the link and sign in to the existing account or create a new one using the link in the invitation email. Either way, the key to successfully completing the process is using the link provided in the email.