I am sorry to hear that.
Do you have a test kit to check the water?
Although the Ich shouldn't bother the ADF
, the medication used for treating might.
Does the Betta look like it has Ich?
Often treated with heat and salt.
Sometimes a sick fish from the store can spread disease. Doing a quarantine for the fish for a couple weeks is beneficial. And sometimes up to a 4 week period if the fish are especially valuable monetarily or emotionally.
But if you look for fish at the store with perky behavior, not clamped fins, not hiding in the corner or at the bottom or top of the tank, any initial observable thinks like Ich or worms in the tank where the fish are located. Trying of course to find fish which are as healthy as possible.
When you bring home new fish, acclimate their water to your water and unless they are really sensitive fish, this doesn't take too long, as you do not want the fish to get chilled.
Getting the tank cycled info in that article link in my signature.
And being able to test your water to make sure that the environment for the fish is safe and as comfortable as possible so they can stay healthy.
At least some ideas for getting new fish in the future.