A friend of mine was/is having the same type of difficulty with his daughter right now. The mom left a few years ago--found a deal she liked better--and the daughter and dad have been 'on their own' since. She will often discuss things with me that she "couldn't tell her dad", and with her permission, I often relay (translate) information that has to go from her to him. I certainly don't have the wisdom of Solomon, but what I have noticed is that she seems relieved that another adult will help. I have been useful to them both in a few situations--'the talk', fractured heart when she and her boyfriend called it a day, what to do or not do about drugs, etc. Basically, she treats me as a surrogate dad, and we both know that the info she gives me is 'top secret' from the whole world--except her father, and nothing is secret from him. They have developed a very close relationship, and gradually I am fading out of the picture because I'm no longer necessary, and that's as it should be. It helps, I think, that she likes me as a teacher, but the major issue with kids is--as it's always been--the matter of trust.
A pat on the back is six inches higher than a 'kick in the a$$'. Sometimes that's important, too. Doing it the way we have has allowed them to communicate without having to be face to face. As a result, they have been able to gradually get face to face, and that is where we all wanted it to go. I hope something in there is useful to you, DSP. And if I can be of help, please say so here or by message.