In May my wife and I will be going to the USA and our 29 y/o Thai niece will be going with us. She will only stay 2 weeks. She has completed the on-line application and has submitted it. I was told she should take some backup info with her to the interview. My question is, what kind of info should she take? She works, so a letter from her employer would be a start. All suggestions appreciated.

any documentation that would indicate a compelling reason for her to return to Thailand, as you said her letter of employment. any assets that she might own, any responsibilities here in Thailand that would compel her to return.

what they want to see is that she will return to Thailand.