Now, Donald Trump is not crazy, per se (and his willingness to engage with any and all media is actually rather winning) but his birther claims are looney and last night Cooper devoted his show to a) disproving them and b) confronting Trump (alas by phone).
In regards to the former Cooper and CNN actually sent people to Hawaii to investigate whether Trump really does have investigators there.  They came up empty-handed.  Though the fact CNN footed the bill to discover that in and of itself is really a testament to Trump's media savvy.  (Also, probably, CNN's struggling ratings.)
With regards to the latter, Trump called into Cooper last night so the exchange lacked some of the sparks former similar interviews have boasted. 
But even still, Cooper did a reasonably good job of holding Trump's birther feet to the fire.  Not that it mattered.   Who needs facts when you've got ratingsVideo below.