It is strongly recommended that you never use the bird in case of unreliable instruments. You don't know which part of the instruments, ADR, IRS is not working. FPV is generated in the IR part of the ADIRS (the combined computers, 3 on board). Only by positively identifying a working ADIRS, you could use it. But the AF447 crew never got out of state of confusion, they never started real trouble shooting.
QRH 34.07A:
- If remaining altitude indication is unreliable:
Do not use FPV and/or V/S, which are affected.