Is there an absolute correct English language, or is it ever-changing?

I was having a discussion with a friend about the plural word “crab” vs. “crabs” and “fish” vs. “fishes.” She said what I’ve heard a lot, which is that certain words are accepted as correct only after continuous misuse. I understand, and agree somewhat, but I think this is how language evolves. I think there are some boundaries, but they’re personal as I think language usage is. Rarely do I tell someone he’s dead wrong about grammar and spelling because there are so many variations. Thoughts?