Hello, I'm Keith and happy to help you with your question.
There is a Double Taxation Treaty between the UK and the USA (and indeed some individual States also), the aim of which is to ensure that taxation is only applied in one jurisdiction, although variations in tax rates may require an adjustment in another. This is achieved by the use of tax credits, the tax deducted in one country being allowable against any liability in the other.
She would as you suspect, be liable to UK Capital Gains Tax (CGT) on any gain made on the sale. However, if this property was her sole or main domestic residence then Private Property Relief will be allowable against 100% of any gain. Husbands and wives have only one PPR between them so be careful here.
The gain is calculated by taking the net selling price and deducting therefrom an acquisition price. The acquisition price is the original coat plus purchase costs plus any improvements say double glazing, extensions, installation of air conditioning etc but not routine maintenance. There is an Annual Exempt Amount of 11K to set against any gain made.
The CGT declaration is made on a SA108 along with the normal tax return and an entry regarding any US taxation suffered and resulting tax credit claimed can be entered in box 37 of the 2014 version of the form.
I do hope I have shown you a way forward in this matter.