Answer:
If this was just a personal account with a DBA designation and not a legal entity, any authorized signer authorization was void at the time of the accountholder's death. No different than a POA. Definitely talk to your legal counsel if this is a substantial sum and you need to determine the exact date on which you were informed or gained knowledge of the accountholder's death. All of that will come into play if you are going to mount any sort of defense. You should be able to pursue any losses that you suffer with a lawsuit against the authorized signer.