As I understand it, all US citizens living abroad must US file taxes each year, even if they have been out of the US for the whole time. However, I think it depends on how much she will be earning in the UK as to whether she will actually have to pay any US taxes.
She will most likely be paying UK taxes anyway if the company is located in the UK and paying her in UK pounds, but I believe that she only needs to pay US taxes if her UK income is above the equivalent of $85,000 (or somewhere thereabouts).
Hopefully someone with more info on paying US and/or UK taxes will be able to answer in more depth
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