The near term impact is that the calendars of the UK U’s don’t mesh with the typical internship/summer job cycle in the US. If she doesn’t plan on coming home during the summers that won’t be a problem, but if she does plan to be home for at least one summer, it is tricky making the calendar work. And for some jobs- paid staff on Capitol Hill for example- internship experience is important even if it’s not with the same branch of government or the same geography.
Long term likely not a huge issue but as I mentioned above, security clearances can take longer for someone who has spent large amounts of time overseas. Part of the focus of the clearance- has this person been radicalized by a terrorist organization or someone trying to actively undermine the security interests of the USA- become a little more complex the more time someone spends with friends, friends-of-friends, etc. who may be tangentially part of a group which will get higher scrutiny.