I just repaired one of my daggerboard from a Nacra 5.2. The top of the board was opened like yours and the foam was visible but the skin showed less damage

The previous owner tried to patch it with MarineTex or something like it but it failed.

I choosed polyester resin to repair mine because I think that my board was built with polyester. Epoxy boards should be repaired with epoxy.

I started by drying the board in the oven at 140F overnight.

I poured some acetone in the foam to wash away any grease. I dried it with compress air

I poured slightly thicken epoxy resin between the foam and the fibergrass to build back the foam. My fiberglass skin was not as badly damaged as yours so I simply clamped it back in place to let the epoxy cure.

After that I sanded the fiberglass skin away with a buffer and I left a taper edge (approx 10:1 angle) I washed it again with acetone and I build back the fibergrass with fiberglass and polyester.
Finally I taped the repair with masking take to push the fibergrass cloth on the board and draoin the excess resin. I am pretty sure that I have a 50% fibergrass content or better. Once fully cured and sanded, I put some gel coat on it.