My gigantea has been through an MP40 twice, the first time it fully recovered in a month and it's slowly recovering this week from getting sucked in last week (through the foam cover if you can believe that).
Yours however, looks different -- my gig never deflated and never opened its mouth. This leads me to believe that something else is wrong with your haddoni and that the injury was not the cause of the nem's demise -- more that an illness is causing the injury.
My only suggestion is to remove the anemone and put it in a quarantine tank and as Minh pointed out -- hope that it didn't spread whatever it has to your other haddoni. You can also boost up the level of iodine, as that is supposed to help. Other than that, I know that folks have tried antibiotics, but I haven't seen any cases of 100% success.
Unfortunately, there are a few people out there who I suspect have a remedy, but don't want to share. This has more to do with acclimating anemones for sale, and since this involves $$$$ the knowledge is kept a "trade" secret.