VAT on Sales

Hi,
Slightly random technical question, I have been approached by a client who has recently registered for VAT but he believes that he can as his customers to pay the VAT on their historic invoices (last 3 months) & not declare this as VAT-able income, I've never heard of this before, and would automatically advise against it, as surely this would have to be declared & the liability paid. Am I right? TIA
Steve