One more girl is only enough if they permanently separate the girls from the boys so that they never have an opportunity to even see each other. It doesnt sound like they're set up for this kind of undertaking.The number of girls they'd need on the low end to keep the flock integrated would be 16 but that still might not be enough.
OP, your best bet is rehoming the girl. Because she's a layer it might be a little easier finding her a home that wont eat her. Boys are harder to rehome and do often wind up as food, and you have the largest number of boys, so keep the boys.
They cry when you separate them because they've grown up together, but they WILL eventually get used to it if you rehome her. It's more cruel to leave her with a gang of boys who will abuse her probably to death than it is to make them feel temporary separation woes. And no, just pulling her out of the flock "when it gets bad" and then putting her back in to repeat the process is not okay either, that's essentially just letting a torture victims wounds heal well enough to sustain more torture.