If she applies as a foreign student for the full course then tuition fees are likely to be around $40k per year. Then there will be living costs and flights.
I agree about the concerns over date rape and coercion on some campuses. I'm doing an MA at an American college right now and part of my course was about sexual abuse. You would be very far away and even a phone call to give out some advice would be hard.
I did my BA in the UK but spent half a year in the US on an exchange. It was actually harder than I expected to be so far from home and I was a very independent person who happily lived hundreds of miles from parents when in the UK.
It isn't just parents. It's having no friends to call up for a chat or to come visit. It's having nowhere to go when campus empties for Thanksgiving and you have cold pizza by yourself. Being the only person who gets certain jokes or likes certain music is hard.
Doing a degree in the US won't help with a visa to live there either. It may provide some good contacts if she wants to work there, but I she returns to the UK afterwards then she will be saying goodbye to all her American friends.
Also, an American degree may not translate well to British qualifications and she could end up with needing further study to get a job.
Would you consider a trip out and spend a day touring a campus? Also, what exactly would she gain from a U.S. Degree that she can't get in the UK? Would she consider working every summer there instead of doing a degree there?
There's loads more to say and I'm happy to answer questions. We live in theUS and are going through this with a niece who wants to come over so I have looked into some of it.