Do you have an alpha hen in the flock? This little squirt is displaying to all the other hens, "I am the boss lady here now!" I would put a saddle on both the cochin and the faverolle, it will distract the cochin for a time, and protect the faverolle from pecking on her back. Also, if you can use identical fabric pattern and colors. If the cochin continues, you will need to chase her around a bit. Catch her when she is bullying and place your hand on her back, while holding her down. Much like a rooster would do. Hold her still firmly but gently, till she calms down. Then, back away quietly. Also, add distractions to the run like hanging plastic bottles with scratch and mealworms in them, large bird suet holders stuffed with fresh greens & apple slices. Set them out a few times a week. It gets boring for the hens so sometimes they get "PMS" (Poultry Madness Syndrome) LOL!!!
I have an ameruacana, she has always been #1 mama in the flock. Sometimes she gets in a tude and will go after my blackstar. I calmed things down by making identical saddles for them. Then, after a few days I had to dominate the ameruacana a couple of times. Not to break her spirit, just calm her down. Now my rooster sets her straight when she gets outta hand! But yes, pecking order must happen...