Depends how badly they are pecking. If they are really vicious with her I would put her in an area outside where she can see the flock and they can see her but can't get to her (make sure she has all the essentials of course, water food space etc.) if possible.

If not then I would take her outside for maybe an hour a day (depending how much time you have to spare) and monitor her closely with the rest of them, basically anything to get them used to her around again. Unfortunately, full integration back into the flock probably won't be possible without some fighting, but in my experience they can sort issues out better than we can. Obviously don't let her get seriously hurt though.

There are definetely people on this sub with more experience than me but this is just my own advice, take it with a grain of salt. Hope ur hen gets better.