There are a number of things that can happen here:-
Your daughter could agree that your son administers the estate and takes no active part in the administration. She can agree to have "power reserved" to her so that at any time she could become involved in the administration but to all intents and purposes your son acts alone.
Secondly, your son and daughter could take the grant in their joint names and then they jointly administer the estate. The difficult is, there could be delays in having the papers signed because of having to post them abroad.
Thirdly, your daughter could appoint someone in the UK to act as her attorney and take a grant with your son for her "use and benefit". The attorney and your son could then act jointly.
(Taking teh Grant refers to getting the grant of probate which is the authority from the probate registry allowing the estate to be administered).