Locked out of Apple all-in-one desktop

My friend has lost/forgotten her password and is unable to open her iMac all-in-one desktop computer. Model A1418. Upon start-up it always opens to the password window, with no other apparent start-up options. This model does not have a DVD/CD drive.

Does anyone have suggestions to open the computer with a workaround or a password re-set method, please?

Thanks for your help.

[Email Edited by Moderator]