While doing research for an upcoming visa run to Myanmar, I was directed to a Wikivoyage article that indicates that Thai citizens can enter Myanmar without a passport:
If you are a Thai citizen, you can get a temporary border pass at the Immigration office about 2 km before the border on Pahonyotin Rd in Mae Sai for 30 baht. All you need is your bat prachachon (ID card).
This would be awesome, as it just so happens I'll have a Thai citizen accompanying me, and I bet she'd have a lot more fun exploring Tachileik with me than having to wait on the Thai side for me to clear immigration (:
Where is this immigration office, and what is the process that she will need to go through to get her temporary border pass?