Invoicing/entering income

I am having issues with a client refusing to give me the invoices sent to their customers through another invoicing platform. They are arguing that the income is already recorded by the other platform (a professional platform and not an accounting platform) and therefore do not need to be entered into QuickBooks. I tell the client that to get an accurate picture of income, we need to enter all income and expenses in QuickBooks. Again, the client argues that the bank feeds showing money collected is enough to track income. I do not believe this to be true. I am still new to QuickBooks and would be grateful for clarification that I am on the right track. Does anyone have any input?