The problem is that people with problems in the detox system, at least a part of them ,cannot expel the mercury from the blood also because lack/problems of sulfur so then they need nutrition with sulfur like cabbage,garlic..or supplements like MSM (I think) to remove the toxins but on the other hand, those people can be sensitive to that nutrition/products and to feel negative symptoms after. So for example if your friend is already feel bad after the cilantro maybe she need sulfur for a while.
I asked myself some questions about the combination of the fasting in the therapy; If there are toxins in the blood stream, that the body has problems to remove, maybe if you go on a fasting the body first try to do something to pure the blood stream and not just add more and more toxins from deeper rates of the body and maybe the body also has his own techniques to remove it and if not,to restore it in safer places..maybe it's also the timing of the fasting-because you can first reduce the rate with detox (chelation,sauna,maybe ncd or chlorella-that me and many others are sensitive to it).
"the mercury can be trapped in the thyroid"- I know that some functions of the thyroid can increase after fasting but many mercury poisoned paople are already have thyroid problems and i'm not sure what happens if you keep on fasting more times or for longer or try to detox it in other way or proper nutrition after cause if the fasting detox from other organs maybe it's still preferable.
I already did a long Water Fasting but it was before I remove my Amalgam fillings and tried to detox heavy metals. I felt better for a very short time but some of the symptoms came back and I have some new from all the process..