I’m not in your situation, but I am married to an American.
I agree she needs to really, really understand the visa situation. Including the responsibilities/requirements on both of them, and her path to permanent residency. Basically, what happens if one of them loses their job, or they split up, what is the impact on her residency status? Though I get that asking someone madly in love to consider that is very difficult!
Everyone above is right to sound the alarm about being able to move back to the UK. There’s two flags I’d raise here. The first is that while it’s hard as a UK resident to bring a partner here, it’s even harder to return with a partner. Second, yes if they have children and the relationship broke down, she’d need him to agree to them moving to the UK, she could not legally remove them without his permission.
If she moves there and things fizzle out quickly, well that’s sad but actually not a massive issue in so far as she would just be able to move home. As a parent, I’d put aside money to be able to buy her a flight in that circumstance.
Big things to think about, but my perspective as someone who has been there and done that is that international relationships mean you have to make these big decisions early on. And of course most of this applies equally to him if he were to move to the UK).
Has she spent any extended vists there? America is much more different culturally than we mostly assume!