@Ady *****
not sure about the UK but Australia has 2 options.
1 is as you said, you can sponsor them to visit (tourist visa)
So immigration look at your finances ect, to see that you can financially support them.
2, Thai nationals can apply for a tourist visa themselves to visit Australia, besides other requirements, the financial requirement is that they can show they have at least $5,000 AUD in their bank account & it has to have been in their account for minimum of 6 months prior to their application.
So from what the OP has written I’m thinking they are somehow trying to apply showing she can financially support herself to visit the UK.