What certificate? For a while, the FAA was giving "certificates" to F/As, but I don't think they do that anymore (could be wrong about that though).
All the airline has to have as far as training records is a record of the F/As training dates, what training was involved (which FAA-approved training course the F/A completed), and records of their tests. If an airline issues some sort of "certificate", it's for company pomp & circumstance, not something that carries any weight with the FAA.