How often does she fly into Thailand? Per 12 month period? Per calendar year?

Frequent entries, and/or a total duration in country, eventually tip a yellow/red flag in the Imm. system, so it is rumored.

A one-day turn may raise different/separate flags with Immigration. Make sure she has 20,000 baht (equivalent cash) and a hotel rez. in hand as these may be asked for. An honest, esay-to-understand reason for the visit might also be handy.

It might be helpful if you detail the "hassle" encountered previously? Was it simply a few questions? Sidelined to a supervisor? Held in a room? Ultimately allowed into the country? How was the "hassle" overcome? Any unusual notations/stamps made in the passport?