Yeah if someone's benefitting from a placebo effect, the worst thing you can do is point out that it's a placebo. If you convince them it won't work, then you've just destroyed the therapeutic effect their brain was giving them. Just shut up and let the placebo do its thing.
Placebos work even when you knows it's a placebo though. Pointing out something is a placebo is important because many are at best overpriced scams (homeopathy) and at worst actively harmful (chiropracty). The culture behind many placebos is also rife with pseudoscience and advocates against seeking out genuine care, so you should ensure nobody gets invested into placebos past a certain point.
One can make an informed decision regarding taking placebos if and only if one knows it's a placebo, else one will be scammed and/or harmed.
Depends on the setting. In a nurse-patient situation, you don't ever bullshit them in the hopes of tricking them into some kind of benefit.
If your grandpa is raving on Facebook about how acupuncture is working better than opioids for post-knee-replacement pain management, then... "Hey that's great you found something that works for you!"
YetAnotherUser makes a good point about not enabling a culture of scams or pseudoscience as well.