@Grams2five
Unless the OP is the exception to the rule I would imagine both parents have unpicked every occasion and conversation they have ever had with their estranged daughter, trying to find what they did so wrong. Is a fallacy that estranged parents suddenly lose a much loved daughter or son and simply say ‘oh well it’s nothing to do with us let’s just blame everyone else’, self analysis comes first. Indeed the first thing most of us do is beat ourselves up, to take the blame and the shame for whatever we might be blamed for. The most asked question parents say to each other is ‘what have we done?’
But if these parents are years into their estrangement they have probably exhausted self analysis and instead examined what is going on with their daughter, and a coercive partner is not out of the question.
As for therapy I don’t recommend this with estranged ACs unless is family therapy, of course that is if the issue of safe keeping is not in question, because one side of anything that could result in permanent estrangement is not advisable, surely all sides should be taken into account. And though there are good therapists out there it has hit the news recently how little therapy and counselling is actually monitored, and suggestibility is probably high with young adults looking for a sympathetic ear.
Just my opinion though so take it or leave it.