Hi,
if the VIES website is reporting the number as invalid there is really nothing we can do. Please see: http://ec.europa.eu/taxation_customs/vies/faqvies.do?locale=en#item_7
If the number quoted by your customer, even after checking, continues to be ‘invalid’, you should ask that business to contact its tax administration to request that the data in the national VAT Information Exchange System (VIES) be updated (only national tax administrations can update VIES data).
Should you find out that your business’ data are incorrect or not up to date, you should contact your own tax administration and ask for it to be corrected. Please note that some EU tax administrations will require you to submit such requests in writing or by completing a form.
Thanks