Let me shed some light on what happened to your client's transactions, malone2023.
When a customer pays, you'll have to record the payment in QuickBooks to close the invoice or reduce the balance. The check showed under Accounts Receivable because it's categorized in the mentioned account. On a cash basis accounting income is realized when the invoice is paid.
For more in-depth information, you can browse this article: Choose between cash and accrual accounting methods in QuickBooks Online. It goes into great detail on how each method functions and how it affects your sales tax.
Furthermore, the links below provide a list of tools that will show you how to organize your sales transactions, manage your company's income, and learn when to utilize cash and accrual basis accounting:
If you have any clarifications or questions about cash basis accounting or any customer-related concerns, drop a comment below and tag my name. I'll get back to help you, malone2023.