Been in a similar situation where customer wanted me to implement supporting fishy, I asked my boss to confirm by email but he never did and told me to do it orally in vague term. I just didn't do it and told them to find someone else, they did find someone.

Always keep in mind that when shit hits the fan they throw your name very quickly and you are going to stand in court to defend yourself. That's what happens when you work on devices which can kill people.

I learnt that they cannot reproach you to stay ethical and they know it is very difficult to make someone do something which goes against his values, it is easier to find someone else.