Upvote:2
If she is honest and the nonimmigrant Visa is refused for reasons of insufficient ties to her country of residence then that sucks but it's not a disaster
If she is dishonest and they discover her dishonesty now or in the future that is likely to be a major problem for your future plans.
She should do as much as possible to show that she has a life in the UK that she does indeed intend to return to (assuming that in the short term at least that is her intention).
P.S. if you want to bring her to the US legally to live with you then I think you will need to marry her.