I suggest to create a single special Apple ID for updating using Apple's App store, create it using the no credit card option. That is what I do for all of our Macs, no having to worry about if I am using the correct password or have the right username etc.
To create an apple ID without a requirement of a credit card, just use the Apple App store without logging in, then find a free app, download it and create an Apple ID. When it comes to payment you will have an option of none, select it. This is the only way to create an Apple ID without a credit card. Any other way forces you to use a credit card. This way its optional if you choose to give the username and password out for the end user to update without worrying about being charged!