We seem to have some confusing advice here the main point is if she has been in the UK for 6 months already she must spend 6 months in Thailand before re applying.
The applicant may apply as the fiancée immediately as this would be under a different criteria however they must demonstrate they meet all the requirements of paragraph 290.
Also the ECO will be looking for them to demonstrate the relationship is subsisting and he has sufficient funds so he meets the maintenance and accommodation requirements.
Regards Paul
