To be honest, I don't think that discoloration happened in just one day. The bird may have been off for a while and it went unnoticed since they do hide illness fairly well.
I suppose it will remain a mystery.
She may want to check her other birds to see if they are developing raw areas on...
That makes more sense.
I actually had asked the question about the location of "injury" in my post too, but then your title says "Back" so I assumed I was just looking at the photo incorrectly so I deleted that part LOL The vent looked upside down to be viewing the back, but then again...
So...