No. It's not true. The harassment will start up again as soon as the sun rises. The best way to integrate is to integrate several birds at once. A singleton has a very tough time of it. These days I don't even try to integrate a single chicken into an existing flock because I have had failures with it in the past. Can you separate Bella into a smaller space with 1 or 2 passive members of the established flock? If they can become a pair or trio it would be easier for all involved when it comes time to establish one flock- you can (re)introduce them all at once and they can share in the abuse. Integration also takes a lot of time to be successful. They need to be able to see each other for a couple weeks, but not be able to interact. Get familiar with the sight of each other, but not be able to beat the stuffing out of each other. Too many people attempt to integrate too quickly and that leads to either complete failure or a beaten up bird(s).
Good luck. I hope it works out for all.