If you feel that her quality of life has gone - it is kinder to let her go.
I have faced this situation countless times - with my own animals, but mainly with my patients.
It would be interesting to know what the diagnosis was. (I expect renal failure).
I am not a fan of the 'Thai way' of letting nature take its course - I think it extends the suffering in many cases.
If you can find a vet who will euthanise her, that would be preferable.