The UK Embassy in Thailand have no part in the decision making process, most are processed centrally in the UK.
She needs to convince the Entry Clearance Officer, the decision maker, that her ties to her Thailand are so strong that “on the balance of probabilities” she would leave the UK at the conclusion of her proposed visit.
The ECO has to make a judgment call based on the evidence she submits, she cannot prove that she’ll leave, just satisfy the decision maker that she’s likely to do so.
She needs to provide details of her living arrangements in Thailand and how she supports herself, you mention a 13k pension is that THB or GBP, and how often does she receive that amount?
Dependants in Thailand will probably be discounted, it’s not unusual for Thai ladies to travel to the promised land and leave dependants behind.
What are her plans for her holiday and how is she intending to fund it.
Is she receiving any financial support from anyone in the UK, if so they would need to provide evidence of affordability and that it’s reasonable for them to do so.