UK Tourist Visa - Lack of ties/finances

Hi I'm Josh, I was born and live in England.

Can anyone offer some advice on whether it will be possible to get a tourist visa for someone with no financial documents?

I have been in a long distance relationship with a Vietnamese national who has recently received a green card visa to reside as a permanent resident in the USA.
I have been to Vietnam twice in the last 6 months to visit but now I'd like her to visit me in England.

She still currently lives in Vietnam but is moving to the USA next month to live with her family. She quit her job and has no bank statements or any ties to the country due to the move in progress which I know will look bad to the immigration officers.
I am planning to be the sponsor for the trip and provide my financials as necessary.

She will only be visiting for 2 weeks over Christmas as this is the only time I have off work for a few more months. She has had multiple Visa's to study in America over the years and has had visa's to other countries such as South Korea and Singapore. This gives her a good travel history but I am worried about her not currently having any job or bank due to the move to the USA.

Any advice on this will be greatly appreciated, I am thinking of getting professional help for the application as I want her here for Christmas no matter the cost.

She has just recently completed all her interviews and been granted the USA resident visa, will this help in any way?
And perhaps she should wait until she's in America before applying?

Many Thanks,
Josh