Waste of a cactus? They didn't waste shit. Give it a couple weeks to scab over and stick it back in the ground. It will start new growth a few weeks after that. Stop and think before you start bitching.
As for what to do when you find a San Pedro, harvest a couple arms but leave most of the plant intact if you can. It will keep for about two weeks before it either needs to be eaten or planted.
no thats not San Pedro, sorry
This is San Pedro:
Whats funny is that when I first started trying to find some pedro I found the same cactus you did and even ate a bunch of it, LOL. Luckily it wasn't poisonous. It wasn't bitter at all either.