So back to the heart of the call from Spouse A’s lawyer. How do you serve a subpoena on an offshore bank?
Short answer: you don’t. At least, not if you actually want it to work. Yes, theoretically, you can serve a subpoena under the Hague Service Convention– in some places.
In other places, various authorities may view your service attempt as a usurpation of judicial authority and reject your request. More to the point, even if the thing could be served, it has no teeth, so good luck enforcing it.
Serve a U.S. subpoena on a Cayman bank, and the bank will rightly ignore your demand. Likewise a Swiss bank or a Jersey bank or Singaporean bank. They’re all going to giggle at you just a bit while sticking your subpoena quietly in a drawer. Why? Well, a subpoena loses its coercive effect when it leaves its own jurisdiction– and it only gets it back if (1) a statute automatically confers it– see the Uniform Interstate Depositions and Discovery Act (UIDDA)– or (2) if a court in the foreign* state blesses it with that court’s authority– as in a garden variety domestication action.
But there is no treaty comparable to UIDDA. And an offshore foreign** court is just not going to rubber-stamp a New York subpoena like a Missouri court would (Full Faith & Credit is not a thing overseas).