I think, as a lot of posts have stated, it's six and two threes.
She (willingly) drank a ridiculous amount of liquid - Her fault
The DJs didn't notify contestants of the risk (as some people said) - The DJs' fault, NOT the company's.
The nurse didn't step in - The nurses fault
The lady didn't go to the hospital - Her fault
She put herself in a very unnatural situation, and when she felt it taking a toll, she didn't go to the hospital. I'm sure most of us would think "Wait, I've just drunk X litres of Y, and I feel bad. Maybe I should go get checked out". There was a nurse there, according to some posts, and she had knowledge of water poisoning. She should have said "Look, go to the hospital", unless Ms Strange refused treatment, at which point it is her fault.
I don't think holding the competition was the bad thing, it's a very... out there idea, but it's not a stupid one. The contestants should be clever enough to realise when they've gone too far, or are in danger of doing so. If they continued regardless, it's their own fault. BUT! The radio station should have had something to cover their own backs, whether a waiver or some form of terms & conditions.
I don't think one person is to blame, but I'd say it's mainly Ms Strange, followed by the DJs, with the nurse maybe taking some, depending on what truly happened.