Hi Arian,
They do can login to the control panel
When you start creating a Customer, Plesk makes you configure 2 different credentials
The first one - Access to Plesk
The second one - Subscription
The first Credentials will allow the user to access to his control panel
After finishing the creation of the Customer, the user will be able to log in
As this user is granted as Owner, he will be able to change his password
Hope it helps!