Resident and tax resident are different things. The link I gave you earlier gives you the info you need from the horses mouth. Nobody said she’d have to PAY taxes in the US, just that she’d have to file them. I earn nothing in the US, all of my income comes from investments, property and a job in another country. But because I spend enough time in the US each year I’m tax resident there, and so I have to file a US tax return. I get credit for the tax I’ve already paid in my home country as there is a tax treaty in place, but still need to file and then potentially pay any difference. It sounds like your GF wouldn’t earn enough to pay tax in the US, but she’d still potentially be liable for filings.
Snowbirds aren’t relevant as they don’t spend enough time in the US. You’re asking about her spending 9 months out of 11 in the US so a very different scenario.
Another thing to factor in is health insurance.