Im sorry but I fail to comprehend how you would know what agents submit what. You have no way of knowing how many applications are rejected daily! There are 1000's if not more, agents offering this kind of service. How could you possibly know this for sure?
Done it myself a few times and no issues. The more documents you can supply the better. You need to prove she will be financially taken care of or she has enough funds, letter of invitation from whoever owns the house she will be staying at and also prove she has good reason to return to Thailand.