I worked in banking, in a department that had the ability to actually adjust transactions on customer accounts. I truly know what it feels like to have somebody get irate because they want you to do something illegal. In my case we had management blessing to tell the customer "No, I cannot accommodate this request because it is against the law. I will have to end this call/ignore any future emails if you persist in asking". After that point, we could bump the customer to our supervisors.
The only thing you can do is keep telling her no. Is your supervisor of any help?
In fact, I would REQUIRE this of my people.
I once asked a company for help with their product in a way that revealed I would be using it contrary to their company's official policy (multiple users instead of just one. Their insurance wouldn't cover them if they in any way ever appeared to condone this. The lady said "I'm sorry, I have to end this call." and hung up.
In your case, you can't hang up on her, but you can say, "I'm sorry, you are asking me to do something illegal. I will have to end this conversation."
Be abrupt like that. Make it clear that what she is asking is DANGEROUS TO YOU and you want to DISTANCE YOURSELF.
Be a little offended! This woman wants to hurt you in order to get what she wants. Show that offendedness. It's completely appropriate.
Smart move, meeting in a neutral office.
You can even say something about the other people present: "In front of witnesses, I am saying to you: what you are asking me to do is illegal, and I will not do it. I'm going to end this conversation now."
Act as though the stakes are really high--because they are.
Also, "will not," perhaps, instead of "cannot," to make it moer personal and clearer.