Betta Fish: Hello my girlfriend took a sick betta from work home. Now we are seeing this spot on its side. What could it be?

My girlfriend works in a petstore where they also sell fish. This betta his fins were eaten by other fish so they couldn't sell it anymore. That's why she took it home so it could heal up. His fins have grown back nicely. Only we are seeing this dark spot on it's side. What could that be? We have never seen that marking on any other fish we have kept.