Make sure she gets her ass into counseling ASAP no matter how little she thinks she needs it. Make that argument. Save her from ending up like this and shorten the after effects as much as possible. Get it out faster the better.

How to respond... Well, I totally suck when it comes to someone I am close to as how to respond, I still freeze and shut down and have with even people here I have grown used to when they go through more than I can think about day to day along with my own shit. It happens and don't beat yourself up. Long as she knows she is your friend and knows you just don't know what to say but are there and understand.

Think about it, there is not much someone uninvolved could have said to make us feel better, and you need to worry about you as this is going to make your stress levels rise. Don't forget you need to take care of you and do not over extend yourself.