Does she wish to and have the right to settle in the UK after school? As you note, most of her contacts will not be American. Some large American firms do interview there, but mostly for openings in theeir European offices. The timing of the school year is a bit problematic for American internships-they are in school later in the Spring and go back to school later in the Fall than US colleges. Also, there aren’t grades until the end, which can be sometimes difficult for some US employers who wish to see a gpa. However, it is very highly regarded and all the major finance and consulting firms will interview, at least for the London offices.
The US government hires interns and FT employees from every school; an Oxford degree would not necessarily help. Grad schools obviously would know the value of the degree.