Hi Carrigon,
I have had some issues with yeast infections in my mouth as well, though not so severe. My wife suffers from chronic yeast infections (she is otherwise very healthy though and shows no signs of ME/CFS). These yeast fungi are part of our body and they only start causing trouble when the balance is disturbed. People taking antibiotics for example or people who have compromised immune systems. As our immune systems are probably compromised, I am not surpised we are having these kind of issues. Chronic yeast infections are also very common in people with HIV because of this.
Antifungals don't always seem to work. That's why all kinds of diets were developed. That's the only thing I can think of (besides probiotics, which are also a good idea) that your aunt could try, a candida diet.This diet eliminates all sugars, yeast, dairy, etc and it gradually starves the fungi. During most of these diets you also take natural remedies against yeasts, like for example garlic pills (though this becomes a bit more edgy as there's not always proof these remedies work).
I personally know people who have eliminated yeast infections by following these diets. I even know one person who has very much improved her ME/CFS condition by following such a diet (this becomes a bit more edgy again as I can't be sure it was the candida that caused her condition, she might have suffered from bad allergies or something else). This is the reason why I am following a diet like this myself right now, though I have had no benefit yet on the ME/CFS side of things... but I am not following the diet very strictly yet. This is because the diet can be difficult to follow. It eliminates many of the fun foods in life... I am having a very hard time eliminating all sugars. I think I have eliminated them for about 80% now during the last month, but the other 20% is so difficult.
If you Google candida diet you will get loads of information.