I agree with
@Tracy B above.
It will involve some sophisticated drafting of both documents*
and a fair bit of collaboration between the lawyers in each country.**
But it's very do-able.
.
I agree with the suggestion to have a local executor in each country.
I would add that, while I would expect the executors to be collaborating,
it may be simple and convenient to keep them separate.
That is, I do not see a need to make them joint executors of both wills.
Your friend may find it convenient to use her lawyers in each country as her executors.
While there may be varying local technicalities in each country, both sets of lawyers
will be working within their scope.
For practical purposes, your friend would be well advised to include in her Instructions to each lawyer
an express authority for each to deal with*** the other.
This may all sound complex and scary, but it's not, really.
It's all about capturing and accurately expressing your friend's testamentary intentions ("her wishes", so to speak)
This kind of thing is not uncommon (especially between Australia and the UK)
and is really just a bit more grunt work for the lawyers in the setting-up.
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* For example, all references in each to the other document,
and to any particular assets, will need to be both express and precise.
** Yes, she will indeed need one lawyer in each country
*** That is, to talk to, write to, and share information with, each other, etc.
Basically, you would be giving them express permission to talk to each other about things that would, in the normal course of events, be strictly confidential.