It is entirely your decision what you do with her. If you are planning to breed your own replacement birds, and you do not want to keep non productive birds in your flock, that would sway your decision towards culling (either by euthanasia, or giving her away as a pet bird with full disclosure). If you merely have a flock for "pets", and you do not mind having a bird who will most likely never be as productive as those who grew normally, then you might choose to keep her. No right or wrong decision in this matter. We all have our own flock goals, and our own husbandry methods.
I have had birds that were slow to feather in the past, but those rare birds have all caught up to their peers by the time they are 4 - 6 weeks old. My most notable bird was hatched with one entire flank that was void of down. She was an ugly chick. Truly ugly. this was my first hatch, and IMO, she "stuck" to the shell at some point during her development. She grew out well, became my most productive layer, but was a moon walker, showing signs of neurological "issues" during stressful times (cold weather, molting). I treated her with PND, and she always stopped the moon walking after the PND until the next time it started.
And I currently have some chicks that are smaller than their peers, but they are making nice size gains. I will be removing some from my flock simply to keep my numbers where I want them, so... their growth rates will enter into my culling decisions.
