reintroducing hens back to the flock

Whenever she is healed up.

 

It's not so much when...as how...and why.
Your question is too simple to give an answer.
Figuring out why she was bullied and fixing that problem,
then learning about how to proceed with re-integration is what's needed here.

More info on your flock, coop/run and details of the bullying event(s) are needed.


ETA: Ah, more info(kinda) here: https://www.backyardchickens.com/t/1076350/bullied-hen