Customers forget passwords. You can set a new one for them in under a minute from their Users tab.
You'll see everyone who can log in to that customer's account, and what role each person has.
Write the new password down before you close anything, because you'll need to give it to your customer. Send it in a different way from your usual email, such as a password manager share, a text message or a phone call. Suggest they change it once they're in.
You can't do this for them. The customer has to do it themselves:
If they type the wrong current password, the panel logs them out and the email address stays the same.
If they've lost the phone or app they use for two-factor codes, a password reset won't help. Turning 2FA off has to be done on the server, so open a support ticket with the customer's login email and we'll sort it out.