Friendship

So recently, a good friend commented to me on how a good friend of hers completely went over the deep end.  She concluded the statement with 'if I ever go over the deep end like this, please be the voice of reason'.

I have, on multiple occasions, witnessed various friends go over said deep end.  I have handled the situation in various different ways each time.

I have come to a conclusion.  What can be done when a friend goes over the deep end?

Say nothing.  Smile, nod, and say nothing.

Please let me know if you disagree or have possibly had an experience in which a friend has replied "why yes, you are right, I AM going over the deep end, thank you so much for pointing it out".