What can you do?
All you really can do is to say that you think its a bad idea, but for the sake of your friendship, you’ll shut up about it now.
If she ever turns on you and says ‘why didn’t you talk me out of it?’, point out that you tried just that, but she didn’t want to listen.
She’s an adult, and ultimately responsible for her actions. If she’s convinced that the market is going to go back up – and she’s among many who think so (at least amongst the majority of my friends, this is the mantra) – then nothing you say will dissuade her.