I would pull as much info together and apply . She will have to go to Bangkok for passport scan etc . Not remember how much sorry . Then the visa fee £150 or was it £105 i not remember . When we applied it was around 3 weeks in total . After going to Bangkok it was 4 days after she got the visa and passport back the next day by courier
Does she have children in Thailand , mum and dad and family . Does she have a job (no problem if not) .
We did it earlier this year and 6 month multi visa gained . We provided many photos , convos from WhatsApp phone records . Money transfers etc . Plus a letter stating I will provide a house and funds etc