Robbie, keep the tank unlit and clean,also use a small treatment of methylene blue to stop any fungal attacks.
If you have live food, use that, it is there when the barb feels hungry and will not decay if it isn't.
If it has only been beaten up it should recover with this treatment.
When replacing it in the tank, just before lights out, feed the aquarium first, let them eat, add the fish, then dim the lights.
Make sure you keep your eyes open for any more bullying and remove culprit.
Alan 104