CA Superior Court case - need CA specific information.

Yes. It is actually pretty easy for a creditor to prove an account stated. To be an account stated, it must appear that at the time of the statement an indebtedness from one party to the other existed, that a balance was then struck and agreed to be the correct sum owing from the debtor to the creditor, and that the debtor expressly or impliedly promised to pay to the creditor the amount thus determined to be owing. The agreement necessary to establish an account stated does not need to be express and is frequently implied from the circumstances. Basically, when a statement is rendered to a debtor and no reply is made in a reasonable time, the law implies an agreement that the account is correct as rendered. The law says that an account stated may be attacked only by proof of fraud, duress, mistake, or other grounds cognizable in equity for the avoidance of an instrument. Therefore, the court will not allow a defendant just to simply argue that the claim or demand was unjust or invalid.

Here is what you have to prove in order to establish your claim on an account stated common count: 1. That the defendant owed you money from previous financial transactions; 2. That you and defendant, expressly or impliedly (i.e., by words or conduct), agreed that the amount stated in the account was the correct amount owed to you; 3. That defendant, expressly or impliedly (by words or conduct), promised to pay you the stated amount; 4. That defendant has not paid you the amount (in whole or in part) owed under this account; and 5. The amount of money defendant still owes you.

Sometimes it does not take much to prove an account - an invoice, purchase order, shipping documents, etc. can be enough. Conceivably, a text message or email could also be enough.