I think what your teacher says about fasting is controversial. For serious degenerative disease there's nothing better for cell regeneration than long fasts. And it's been used by healers since the time of Hippocrates.
For all Gandhi's near death fasts, they say his body was shockingly young and fit into his 90's.
But she's right about the danger of toxins while you're fasting. I remember conventional wisdom in the 70's was never think about doing extended fasts without daily enemas to remove toxins. Yet I never hear that mentioned on the fasting boards here. I posted about that and got no response. Gerson requires coffee enemas even for juice fasting.
Yeah everyone needs to take everything they read with big grains of salt (and not just on the internet). Fortunately with natural healing you seldom hurt yourself by experimenting. Stuff usually either works for you or it doesn't. And if it works you can usually feel the difference.