Short answer, as in all things in Thailand. It depends. I've been stopped and they didn't ask for IDP. I've been stopped and they did ask for IDP. 95% of the time just waived through. My motto on such things. It's better to have and not need. Than to need and not have.
The kiss of death is to say she has an American boyfriend. Absent that, she needs to be reasonably well off, with a very good job, own a house, own a business... Even then, they will probably be suspicious of a young lady visiting the USA alone.
Does she have Dual Citizenship? Lao/USA? Did she renounce her Lao's Citizenship? Can she get a Lao passport? If so, go home to Lao, get a Lao Passport. Enter Thailand with Lao Passport.