Dave Wilson, I am not a lawyer!
One relevant point that has been missed is that when pilots are task saturated the very first sense the brain deletes is hearing. Your ear hears but the brain says "hey we're busy enough here - we don't need that stuff!".
So it's possible the pilot is not aware that the stall warner is operating (how many have landed gear up with the warning horn blasting away?).
Similarly the pilot may not be aware of your airspeed prompt. You might have to break the pilot's pattern by (for example) shaking him on the shoulder etc.
In the event of an accident with injury etc IF the passenger(s) can prove negligence on the part of the pilot they would have a case for civil action. But being negligent implies a deliberate act rather than a "genuine" mistake as I understand it but, as I say, I am not a lawyer.
But this is really getting highly hypothetical now. Sounds as though the OP was flying with someone who was not that competent.
One question I often ask students/pilots is "Can I stall if I am diving vertically towards the ground?" (with model a/c in hand pointing vertically down!) The answers are quite illuminating since almost exclusively all their stalling training has been done from more or less level flight and/or modest bank angles.