Personally I would get this guy indoors and start treating him or he wont see the winter out, he appears to have a bad infection of some sorts here..
I know this fish is a friends koi but you should never put a fish from even a friends pond into your own, as you quite simply dont know what is wrong with him and your risking all your own fish too especially if this is fungal as fungal spores will be released into the water, personally direct intervention is needed here .
Never have I seen infection that has gotten a foot hold on a koi like this one but we have to ascertain what your friends koi has wrong with him ........
I'd hazzard a guess but dont like doing that especially as I cannot look at the koi up close.
First off we have the physical damage that the koi recieved whilst out of the pond can you describe what that was like when you got him ?
Secondly we have to look at the growth and my guess is the fussy white stuff isn't protecting him because the mucus layer would have been damaged whilst he was out of the pond........
It looks to be a bad fungal infection caused by the Physical damage.
First off you'll need to treat the damage caused by its spell on dry land or you'll be wasting your time and all your efforts to fix this guy wil be blighted by your attempts .
You'll need to put the koi under anesthetic cleaning the whole site with a cotton buds clean one area ditch the cotton bud and so on till the would is clear, then its best you cover the area with Malachite green and finally spray the wound with Propolis ( you can buy that at a health food shop they sell it in both spray and liquid), its cheaper doing it that way than going to a koi dealership who mark up the price for maximum profit.
Now if when you have cleaned the area and it looks ulcerated.... then this means you have a bacterial infection going on as well this is where the vet comes in you'll need antibiotics Baytril is the one we use either inject or feed at a dose stated by the vet ,.
Use the spray to cover wounds after treatment and the liquid as an additive with your koi's food
Have you ever anethatized a koi before ?
if you dont have any then I suggest using Oil of cloves at 10ml per 2 gallons imperial use more if the koi wont go under straight away.
Watch the koi till it goes over on its side lifting the koi out every few minutes to see how for under he is , once the tail stops moving get the koi into a damp bath towel , you have about 5 mins or so to treat your koi, so have all what you need to hand i;e cotton buds ( as much as you will need to clear all the infected areas up ) malachite green and propolis working towards the tail clean away as much fungas as you can then paint librally with the MG making sure you dont get any on the gills.finish off with the propolis spray return the koi back to water and hold the koi gently in a current with air bubbles till you feel strenght come back into the koi..
rgrds
Dave