First of all,
Washington DC is an amazing place and definitely a go to place for study abroad.
The advantages of a study abroad is that it looks really good on the CV, great talking point in graduate scheme job interviews and your DD will learn a lot of life skills and experience from participating in it.
Generally I believe the work will contribute towards her degree but depends on her University! I remember when I considered a year abroad in America, they had 4 modules per semester so it meant more work. Also the way they were assessed was different. In Scotland, at my university you got three modules instead of 4 every semester but also in America they had weekly quizzes, mid term exams and final exams where as in Scotland you only have final exams.
I suppose if your DD is very hard working, gets good grades, this all should not be a problem for her though. In terms of costs, most of the cost will be covered by your DD. I believe she will not have to pay tuition but the cost of accommodation, food, spending money will be on her. However, if your in Scotland and get SAAS or a student loan in England (I am not 100% sure about England).
The best thing to do is go to the website for the University she intends to study at. Some students find a semester abroad instead of a whole year a lot cheaper. Sometimes the rates of accommodation on campus are not that bad as most US universities do include meal plans so her food and board may be covered.
Generally if she can overcome the financial challenges and be sorted that way then I would 100% recommend it.