There are two ways to see if a customer has a login.
1. In their customer details account, if they have created a login it will say so with a little green flag directly below their name.
2. You can also see the entire list of Customer Logins from your Customers page.
As an Admin, you can change a customer's email address from their customer account.
How to update your customer's email address:
Navigate to the customer account
Select the Info tab
Once in the Info section, press the Edit Info button
Update the email address and Save
The customer will now log in with this email address