Did you actually see the fish damage the Oranda's wen?
Any time a fish is injured with any sort of "hanging parts/skin",you have to really seperate them.I had a Bubble eye that popped his bubble,and he had the sac hanging loosely around his eye.I swear that every other fish took a bite of it.I think it's just natural if they see something sort of" freeswimming" past them,to automatically assume its food.
The thing you want to be aware of is,was it really a fish biting the wen,or the beginnings of hole in the head disease,which has been quite a popular disease this year for Orandas.
Pristene water is of the utmost importance,and seperating him was a good idea.
What are your actual water param readings?
As mentioned,the shredding could be part of the original damaged wen,but should not continue on.I personally would feed some medicated food.I don't normally like to automatically dose with meds,but I do know that when it comes to wens,it can change for the worse overnight,so I'd personally feel safer feeding the med. food,as mr b. stated,more preventative.Keep an eye out for any unusual behaviors, any redness or new deterioration to the wen as that may be a secondary infection setting in.
Since you've allready done the Peroxide,I would not do that again.If you feel the need to do anything with the wen itself,you can smoothe a dab of neosporin on it.How much more wen has he lost since the original damage?
As for the Melafix,I know alot of people think highly of it,but I've found personally that I have just as much good response using perfect water and sometimes salting to 0.1% depending on what is actually going on.