She can live in the US for 6mos with no visa.
She absolutely cannot do this, assuming she's a UK citizen!
She can visit for up to 90 days using the Visa Waiver Program (completing an ESTA before departure). There is a 6-month tourist visa you can apply for in advance (B), but it is rarely granted to working-age people who could use the VWP, and once you get rejected then you're screwed because you'll have to declare the refusal on your next ESTA application and that will probably get you rejected.
You're not supposed to enter the US as a tourist with immigrant intent and then can be really hardline on this if you have a partner there - if you get flagged for secondary at immigration (which you may, after repeated visits) they'll check your phone, read your social media accounts, and any hint of 'I'll go over to the States and maybe think about staying' is going to get you bounced, which will then affect your ability to visit the States ever again.
Also, most US work visas allow you to bring over a spouse, but the spouse may not be able to legally work, or has to apply for an Employment Authorization Document only after they've arrived (L, J etc).
Why rush into marriage with someone who's already shown their priorities are elsewhere, then move overseas to be financially dependent on them indefinitely or at least for a few months, thousands of miles away from your friends, family, commitments and career? (That's not really directed at the OP, who seems completely clear-eyed about the situation; more at this mindset that it's romantic to chuck over your own life for some guy who tells you via text he's leaving.)
Also, it's crazy to compare this to WWII: people who left home then were either drafted, or believed they were doing their duty for society. This guy is not off to fight the Nazis. He's doing what's best for him, and that's fine, but he shouldn't expect the OP to make any sacrifices at all.