I don't see any significant damage or any inflammation or redness on the IDS in the photo.
I don't understand the "water conditioner is a medicine" statement, sorry. Just new to me. Never heard of it. Can't enlarge the picture to see what the label says... The most I have come across is that water conditioners state "helps restore protective slime on fish" or some such, or once I came across a water treatment like a bag buddy that stated to "help start healing minor abrasions faster", which may be related to the previous statement.
IDK what head rotting is, never heard of it. Sorry. I am guessing hole in the head disease? If so, HITH is a very tough issue that requires a serious approach.
Anyhow, the main thing, as stated above, is that the affected fish is comfortable and stress-free and it will recover on its own just fine, chances are. Medicating is the last resort in my book. Other peers may think differently.