3yrs ago, I visited on Esta, got married, filed CR1 and left. Total stay was 85days. I know the CBP environment is now different, I’m sharing my experience in case it helps.
1) Be ready to answer questions on reason for visit. I answered truthfully that I was visiting to tour my future country, get married, file for CR1 and then leave. I also informed that I have a return ticket on xx date. Esta is for tourism, not for living in the US, hence I emphasized I will be doing some tourism.
2) Be ready to assure CBP that there are no plans to adjust status and your gf’s previous clean visit records show that she plan to handle her migration the rightful way. Also ensure she knows your house address by heart.
I had left my job when I did my Esta/marriage visit. CBP did not ask about my work, I think they are more lenient towards Esta visitors.
On a side note, she can consider applying for Global Entry with her Japanese passport.