Lady Galena, getting so emotionally attached to your former slaves will likely do you no good, such as we can see now.
As a freewoman Serena is able to make her own choices, and one of them appears to be associating with a grave heretic. If it is proven this was not a choice of free will, and instead coerced by other means, then I am willing to change my mind on that.
Sadly, as it stands, it appears that she has seen the door open to her and slammed it shut by her own hand, trying to ‘save’ her now would be an exercise in futility. Heck, even if you were considering chipping her it would not undo her actions.
As my late Uncle might say, she’s a write off. Better to move on unless the opportunity comes up to put her down.