I guess in many cases the problem is that it is not obvious how to react to mental health issues. There are only theories on how to cure someone, but few certain measurements. Still, at least attempts at cures should be made (or at prevention).
As their family member it's obvious how to react: tell them to get help, and ride them about it until they do. I've done this. The second part is probably the more important.
There's no guarantee that help will be successful of course, but you've at least done what you can.
