Re: Bristish girls engaged/married to American guys...
Travel between the US and UK is visa free for the most part. My husband and I go back and forth every couple of years and have never needed a visa. The girl will need to apply for a fiancee visa or have her husband sponsor her the same as anyone else, though. If he sponsors her for a green card they may need to prove that they have/are living together and are actually married. I would recommend that she return to the UK and apply from there, as the wait times will be shorter and the embassy staff tend to be more helpful.