Anubias grow under pretty much anything. They are pretty bullet proof as far as plants go. How long ago did you add the medication and how many water changes have you done since? Also how long have you had the plant? Some times the shock of moving from one aquarium to another can cause the plant to melt but they always normally grow back. I also second the rhyzome thing.
There is no such thing as ‘Rescuing’ a betta or any fish for that matter from any LFS. Be it petsmart, Petco, or exotic and aquatic’s down the street. You are not ‘rescuing’ anything, you are buying a fish. It doesn’t matter if that fish has fin rot and you are saving it from dying a slow death, you are still supporting an industry that thinks it’s okay for these fish to live in small pint size bowls.