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?

Visits and duration... Since I've known her she has done 4 visa runs per year for the past 5 years with max 3 day stay - not always to Thailand., but it's usually the cheapest.
Reason for visit... How 'bout the truth - visa run.
Funds... 20,000 THB? For 1 day? US$661? Must be typo or 1 month stay. How much for 1 day?
....yikes!!! I see "Thai Law requires tourists to arrive with a minimum of 10,000 Baht (~300USD) per person". That's outrageous!!!! And not possible. Other sites say "Backpackers should budget 800–1,200 baht a day ($35–$50).
Hotel rez... $5 dorm (part of "funds available"...?)
The "hassle"... She said "they were curious why I always came to Thailand every 3 mo for over 2 years". That's all. And it seems to have been resolved amicably
Unusual notations... I'll ask her to check her passport - all of it - and expired ones too.
Also... her passport was renewed in Vietnam - don't know what year but it expires in 2021. Prior to that she must have renewed in US or Canada,