I'll share with you some information and troubleshooting steps that you can perform. So, you'll not be entering any password while opening the client's QuickBooks Online Account.
Normally opening a client's account in your QuickBooks Online Accountant Account (QBOA) shouldn't ask for a password. Then there shouldn't be any problem opening the file.
With this, I'd suggest letting your client re-invite you as an accountant to get rid of the problem. Let them click the Delete or Resend invite button on the Manage Users page and sign in with your existing user ID and password.