Live and Let Live?

Yesterday my wife and I had a rather awkward situation. While eating lunch with our daughter an acquaintance bumped into us. In the course of the conversation we told her that Rachel has had a stubborn ear infection. This acquaintance then proceeded to go on for several
uncomfortable minutes criticising mainstream medicine and encouraging
us to go visit a chiropractor for her ear infection. I was as
respectful as i could be, but politely said that some of the "adjustment"
procedures used have been proven to be very dangerous especially on
children and that we would stick to what our doctor recommends. Her
family honking for her to hurry up and end the conversation was a
welcome disruption. I was having a good day and just didn't feel like
debating whether or not chiropractic was as real science or not in a
Chick-fil-a playground.

For the most part I take a live and let
live philosophy when it comes to others' beliefs, no matter how far
fetched or different from mine that they may be. Unless I can see any
real damage or harm that could come from their believes I tend to
respect them. It is only after they feel the need to "convert" me totheir beliefs that I feel the need to criticise them.
Now here comes the rub. At what point is it acceptable for me to challenge somebody else's
faith that I don't share? Perhaps it's a Christian Scientist friend
whose child is very sick from something that a simple treatment of
penicillin would cure. Or maybe a Jehovah's Witness who is refusing
blood products. Or a friend is recommending that somebody forgo a
surgical procedure for cancer in favor of homeopathic treatments. Or
maybe it's simply resorting to prayer instead of other treatments.

All of these are fictitious examples, some more than others, but I gave them just to illustrate the seriousness of my real dilemma.

At what point is it immoral for me to sit back and not voice my opinion
and try to help out, knowing that it will challenge their faith and
potentially our relationship?
I have yet to meet anybody who agrees
entirely with my philosophy on everything and I personally hope I never
do. What in the world would we talk about? But I am curious about when
it is appropriate for me to voice my concerns about others'beliefs and when it might be more moral for me to not antagonize my friends and family.
Like
the example I've shared this has become an issue several times for me
in the not to distant past. There were times when I stayed quiet that I
wish I had spoken up and there weretimes when I spoke up that I wish I had just not interfered.
Advice?