Is it possible to obtain power of attorney, when both parties agree. the account holder is in custody in another country,
That a person is in another country, in or out of prison, would have no effect upon his being able to grant someone power of attorney. Afterall, that would be a situation in which giving someone authority to act on their behalf makes the most sense. But I suspect there is more to your question then that. What is it you really are wanting to know?