If she is only 16 years old, please don't tell her to hide supplements from her parents. She needs to have a close relationship with her parents to get through this, and they need to understand how debilitating it is. Their #1 priority in life right now should be finding the best diagnosis treatment for their kid. This might be life-long antifungal therapy that falls short of a cure, or perhaps something else entirely.
There are 461 supplements sold in Amazon's "Health and Personal care" section under "candida". Most of those have never been shown to work at all, not even in animals.
GSE, olive leaf extract, and Candex have not been shown to work even in animals. However, these substances are potentially irritating to the intestines.