Sorry to hear you're going through this!
My husband's been dealing with an obsessed alienator, for about a decade. Thankfully, his son never developed full-fledged P.A.S. (which is when the kid completely rejects the targeted parent), but due to his ex-wife's efforts at alienation, DH was given sole custody almost 3 years ago. That solved a lot, just in terms of DSS being able to witness - in everyday life - that DH doesn't "abuse" his wife or any of the kids he lives with, and that his desire to be involved in DSS' life is genuine (not "stalking"). But, if anything, the ex-wife's efforts to influence DSS have become more intense and less subtle, since the custody change.
When I have more time (tomorrow?) I will write back with some good links about P.A.S. Sadly, a lot of what's on the internet about it is just junk. But for the moment, here are some thoughts:
1- Thanks to politics, accusations of "junk science", accidental misuse of the term (such as saying the alientaing parent has P.A.S., rather than the child); and the intentional misuse of the term by genuinely abusive parents who would prefer the court believe their ex is turning their kids against them... there are a lot of courts in the U.S. that simply don't want to discuss P.A. So it might behoove you to research other terms that mean the same thing - or to have your atty. research how "P.A." has been received by the judges in your area. For example, although DH's custodial evaluator did use the term "alienation", the judge who gave my husband custody opted for the term "negative stereotype induction", in her ruling.
2- You must get a custodial evaluation. If at all possible, spend what it takes to get a reputable, experienced evaluator. Petition the court to order the evaulation. Then your ex can't deny permission and he'll also have to pay for half of it (most likely). Sometimes CEs also counsel kids, so maybe that would be a good counselor to choose for your daughter. Investigate your options. If the CE recommended counseling, it would be pretty easy for you to get the court to order it, over any objections your ex might have. A custodial evaluation will involve you, your daughter, your ex and possibly anyone else who lives with you guys meeting with the evaluator, individually and with the child. It will probably also involve you and your ex taking the MMPI (personality test). I've read that parental alienators have a higher tendency than average, to score "paranoid/hysterical" on the MMPI (that corresponds to a numerical score, I just can't remember what it is). If your ex wound up with that result, it might be worth ferreting out that research for yourself, to take to court. But bottom line, it's hard enough getting a court to deal with P.A. issues. You need professional back-up.
3- Be careful, handling your child. Toe the line between correcting misinformation that she's been given and NOT bad-mouthing your ex in the same way you fear he's bad-mouthing you. BOTH of those are important. You don't want to let her believe things about you that aren't true and just hope she'll see the truth for herself, someday. But, if she's already siding with your ex, you don't want to push her away further by making her think you're attacking him. Not to mention, you don't want to mess with her head the same way he does, just to defend yourself... and that's easy to do! You really have to guard against it. For example, you may be forced to tell her something she's heard from her dad isn't true and to tell her what really happened. The next logical step in her mind might be, "Then Daddy LIED?" Well, if he DID lie, then it's not YOU planting that thought in her head, by simply telling her the truth. But you can stop there, and just say, "It must be confusing for you, to hear two different stories from your parents. I don't know what caused your Dad to say what he did, to you. It's OK for you to ask him that, yourself. All I can tell you is what what I truly did and did not do. I love you and I care what you think of me. So I don't want you to believe I did bad things, if I really didn't."
4- One simple, effective way to counter-act P.A. is to take pictures like crazy and spend time making cute, little albums, scrapbooks and slide-shows that your daughter can carry in her pocket, or her overnight bag, or watch on the computer, when she's home OR at her Dad's, to remind her that she really does have fun and feel loved, when she's with you. Of course, that means you need to keep things as positive as possible and get out and DO things with her, even if her behavior hurts your feelings. This is such deeply confusing, difficult stuff for kids. You have to work hard to let it roll off your back. I'm not saying that flippantly! I know exactly how hard it is. I have gone to my bedroom to cry more than once, over something my step-son has said or done that was influenced by his Mom. And he's not even my own child. And the ways that he acts out have been so minor and so few-and-far-between, compared to kids who really have P.A.S. Hang in there!
5- Oh! Also keep in mind that kids don't get to CHOOSE who they want to live with, when they reach a certain age. The court is required to FACTOR THAT IN to the decision, when the kid's old enough. Especially if there's an established history of P.A., a judge certainly has the discretion to conclude that the teen's stated preference is heavily influenced by the chosen parent brainwashing her to reject the other parent and that it's not in her best interest to follow her stated wishes. There are definitely parents out there who have custody of a teenager who swears they want to live with the other parent, but who was overruled by the court.
One woman in a house full of men: my soul mate: or... twin sons:(HS juniors) ... step-son: (a freshman) ... our little man: (a kindergartener) ... and there is another female in the house, after all: our.