Options are:
1. To be in a Tourist Visa - you need to know the requirements to apply for this - reason, money (savings) etc.
2. Fiancee or Spouse Visa - if you plan to get married already... There are members here who underwent the same situation, please try searching in this forum...
3. Work Permit holder - again, this is a different one. She needs to comply with the requirements before she could get one...
The British Embassy website in Thailand is very helpful and I recommend her to browse it.
