Person 1: "I like McDonald's burgers"
Person 2: "Try this burger I made myself from 100% real steak"
Person 21: "Mmm... that is so much nicer!"
I thought I'd start with some things you might want to consider about "alternative" medicine as these might help you get your friend's point of view a little better.
Firstly, even if you believe alternative medicine is useless (as do I, for the record), it is only really "dangerous" if it replaces conventional medical practice. If the patient views it as "complementary" as many do, then it is usually harmless, provided that there are no active ingredients which would interfere with the conventional medication they may be taking. In the case of things like homeopathic remedies, it is scientifically proven that they have no active ingredients whatsoever and are nothing more than sugar and water. The danger comes in when people place false hopes in alternative medicine and disregard their conventional treatment.
The "placebo" effect is also not to be overlooked. Most drug trials include a selection of patients on placebos and the percentage of disregard on the final results for the placebo effect is often higher than you'd imagine - in some cases nearly 10%. I can't explain it - perhaps when someone is mentally fighting something they are physically stronger? My point is that even conventional medicine accepts that there are some positive effects from patients believing they are getting help.
Of course, the internet is full of people who claim to have got better from this kind of treatment alone. But if there are around 10% of people who get better from the placebo effect then these cases can be explained that way. In a clinical trial, the other 90% of people got the right treatment or will get it eventually because they didn't choose a placebo. In real life though, the other 90% will not get better at all. If I was given a medicine that offered a 10% chance of survival I would not be optimistic.
Given the above, if you think that your friend's choices may enhance her recovery or wellbeing even a tiny bit then perhaps you can find a way to be a little more supportive, providing she keeps up with her regular therapy. Perhaps something like say:
If you think that your friend might be open to some reasoning and could change her view, perhaps a different approach might work. Instead of attacking what she already believes, try to put in place new, well-supported ideas that are stronger. People will hold dear to what they already know and believe and defend it; however, when they accept something new that conflicts with what they already know then they are more likely to disregard the incorrect idea themselves.
For example, here is someone attacking another person's belief:
But what if they took a different approach:
This approach may help you - try to introduce your concerns as "new" thoughts rather than attack her existing beliefs. You could say:
Possibly one of my favourite websites ever in the history of the internet is How Does Homeopathy Work? Once you get past the brilliant first page there are links to some scholarly articles and also an experiment conducted by the group behind the page. In one experiment, 350 people simultaneously "overdosed" on homeopathic belladonna (a poisonous plant) to prove that, while belladonna may have been used in the preparation of the product, there is absolutely no trace whatsoever of any active ingredient in the product after the preparation. Scientifically it cannot possibly do anything.