It's a SINGLE entry visa for a maximum stay of 60 days and even if you leave after just one day, it expires. Simple.
Now, you can extend those 60 days at any immigration for a further 30 days at a cost of 1900 THB. Same as before, the moment you leave, it expires as it's just an extension of your SINGLE entry tourist visa.
Now you could get a re-entry permit but that doesn't make sense on a tourist visa. You will not get more days. Just exit the country and come back and get an other 30 days.
PeeJay Ess - You mean: "You will have to take care of your 90 day report as usual until the time you're here on your THAI passport." ?
If she re-enters on her US Passport, she will need to report again every 90 days but not if she enters Thailand on her Thai Passport. Wouldn't that make more sense? Don't know.