The clues are in the rejection letter.
But make sure she has at least
*****
baht going in and out of her account every month.
3 months statements.
Another
*****
0 baht in savings.
A letter from her job or self employment.
A letter from you soonsor saying you will support her.
3 months statements of the sponsors showing £1500 a month going in and out
A ststement shoejng £5000 in savings
Passport