Any subpoena requests are made upon the UK Govt on behalf of its citizen, the "requested party" Our 'Treaty' with the USA outlines that the "requested party" (HMG) may refuse to proceed if "the Requested Party" is of the opinion that the request, if granted, would impair its sovereignty, security, or other essential interests or would be contrary to important public policy".
Unlike members of the Foreign Office serving at the US Embassy, HRH does not possess 'Diplomatic Immunity'; but it could be argued that in coalescing with the service of said subpoena a conflict with sovereign 'process' could be encountered?
So far Andrew has made a public statement saying he was "willing to help any appropriate law enforcement agency with their investigations". This indeterminate statement is not sufficiently specific within its content to presume if this extends to a personal appearance (at enormous National cost in security provisions etc); to aid many would say (passim) a fallacious attention seeking action, so typical of American litigation? I would certainly advise him to only submit written information in response to questions 'in camera'. If pressed he could also provide a video link perhaps? Formatively, assessed none of this improves his situation and I would urge rejection.
Public opinion majoratively portents to interpreting lawful process by the expression of emotion, within a conflict of definition and understanding to same?