Amongst the limitations 'applicable in case of in-flight landing gear retraction failure', the A310 FCOM Chap 2.18.40 (Rev 30) Page 1 does indeed contain this statement : Fuel predictions of FMS must be disregarded
Perhaps this was introduced after the event? In this litigation-hungry era, I'm not surprised that Airbus have had to spell such things out in order to cater for total idiots whose slavish obedience to the book have displaced any airmanship they once had.
The F/O should certainly have spelled it out to the Captain and backed up his concern with accurate fuel measuremens. Didn't it occur to either of them to cross-check the Fuel Quantity Indicators, ECAM fuel page and engine fuel flowmeters in order to resolve the clear discrepancy between FMS and FQI which must have been manifestly obvious? And why did his 'Company' not think to suggest it??