Territorial behavior in red eyed tree frogs generally occurs during mating season or in times of resource (space, water) shortage, and even then it usually stops short of physical confrontation. It's very unlikely that bullying is responsible for the condition of this frog, especially if you have ample space in the tank. However, separating them might be a good idea at this point, if only for quarantining. But make sure the new enclosure has all the right conditions with regard to humidity, temperatures, etc. It also wouldn't hurt to have a vet at the ready should things take a bad turn.