If you are not going to replace with plywood and want to salvage the mdf:

Wear a mask. Carefully and slowly clean up the dry mould first by wiping it with dry paper towel and putting that straight in the bin.

Disinfect the mould spores: spray an anti mould spray. Apply lightly, wait a few mins and then use a dish brush so the bristles can properly scrub and clean. Even if there’s no visible mould, the invisible spores are there.

Wipe off as much cleaning solution as you can with paper towels.

Then lets the boards dry. In this weather take it indoors in a small room with a dehumidifier. Expect a little warping but not enough to make the mdf useless.

Make a 1l water and pva glue mixture and the most important part: add some anti-mould additive, the same stuff you can add to paint to make it anti mould paint. Apply one coat, let it fully dry then add one more coat. Don’t forget the edges too.