Oh man that stinks. Mdf makes lovely mouldings until it gets wet. The only repair is to remove it and replace it. Nothing will make it go back to how it was.

Hallett is correct. It's toast. Replace it with solid wood after you've fixed the leak please.

Remind me why mdf is used in the first place if there is ANY WAY it might get wet?

I had miles of trim in my last house that was MDF with no problems, but my windows didn’t leak.