For the symptoms of many illnesses, knowledge of placebo effect renders the treatment ineffective, and in things like depression, learning of the placebo effect not only undoes any good it had done, but sends the depressive into a worse spiral than before the treatment.
Therefore, you would have to admisinster placebo without the patient knowing that's what it is. That's called "unethical".
Also, in very many cases, you could be masking symptoms of an ilness which needs to be treated. Homeopathy might make your symptoms go away for a bit, but it hasn't treated the underlying cause. In the case of a cold or a phantom illness or injury, that's fine, but for everything else, it's not fine.
I think you will find the vast majority of homeopathy patients are not aware of the placebo affect and/or do not attribute their diminishing symptoms to it. This is confirmed by the fact that homeopaths and the homeopathy industry does not market its products and treatments as placebos, but surrounds them in mysterious claims of magical efficiacy.
Homeopaths will claim that their medicines can treat causes, not just temporarily banish symptoms.
Why not tell your friend to create his own voodoo ritual at home to banish his symptoms? He could save a fortune and give the money to cancer research.