Preventive Treatment - The addition of Maracyn 2 should be standard practice as it will help prevent opportunistic infections from gaining ground on an already stress weakened fish. Maracyn 2 also adds vitamins B and C to the water which may increase appetite and aid the immune system.
Alternative Formalin use : If you feel the infested fish may not be able to handle long term dip methods, you may want to consider using a combination of both dip and long term bath methods, in that, give the fish a one time dip treatment to knock down and remove the infestation and then place the fish into a long term bath treatment to kill any remaining parasites. I would only do this with a heavily infested fish that could be considered a sensitive species or one that is already weakened by the parasite.
I would say, from what this article says, you should treat right away. That "might" help you save this fish. The sooner the better.