Based on the photos yes she removed the top surface of the deck board that has the texture. It is technically ruined. What would I do? I would try a really rough (30 grit?) belt sander to remove the remainder of the top layer and create a consistent surface. Can’t make it worse.

Unfortunately, yes but you can do what the prior post said or get a professional to help. The deck is still functional though - if you can't get it fixed, you can put a nice outdoor rug on top of it.

Put an 80 grit abrasive on a Festool 150 sander on grind mode and see if you can abrade the marks away.

Edit.

Water pressure is a powerful force, isn't it? It can cut right through some materials, if the pressure is too high. I'm sorry your daughter had to learn this the hard way.

I would absolutely put a rug over it.

If it is a small area I would suggest using GN Builders idea of putting down a rug.