That’s what she will have to convince CBP of.
VJ veterans know that more often than not, “circumstances change” and the K-1 applicant decides to stay and adjust status. The OP’s gf has clear immigrant intent.
For those saying their own experiences of traveling on ESTA during the petition phase of the K-1, hopefully you see that this is a different situation. My (now) husband did as well, but he had the ESTA/B2 prior to our petition filing, and he had very strong ties to home.