Compulsive lying

TL;DR: Boyfriend's daughter lies every time she speaks. Why is she doing it? Is there any hope she'll ever stop?

I'm autistic with ADHD, and so are my kids, my whole family, my partner, and his kids (we move in herds, after all).

I'm concerned about my partner's 13 year old daughter. She went from rarely being able to see her dad (due to her mother's obsessive, restrictive schedules) to living with him full time with no contact with her mother, so I understand that there's likely trauma involved. She's recently started therapy, so I suggested he could raise our concerns with her therapist, but we need guidance, too.

I often talk with her, but she lies EVERY time we speak. I don't even need to ask her a question for her to start telling me blatant lies. She'll tell me about her day, and within the first 2 minutes she comes out with something implausible, and that's when it feels like a huge chore and a mental drain to keep on listening, which I feel awful about. I just can't seem to force myself to get invested in a conversation when I know that none of it happened.

When talking about drama that happens at school, she puts a lot into demonising the other students in ways that aren't believable. She often claims that she has recordings of students admitting to things (usually the same things). She then talks about turning everyone against people she doesn't like, and how it's ok because "nobody likes them".

When she's upset because of fallouts at school, she says it's about people starting rumours, and her getting blamed for them. They are rumours that have her signature story style all over them.

Some examples:

I went for a short walk with her over the weekend and she told me that on her previous walk, she was greeted by families of foxes and rabbits and that the rabbits stayed put while she stroked one of them for 5 minutes.

I told her a story about one of my family's experiences, and she immediately repeated the exact same story back to me with the names swapped out for her friends' names (the story wasn't the kind of thing that happens twice).

One time in a fun conversation, her dad referred to a unique, creative insult he made up then and there. She immediately claimed that she and her friends had been calling each other that name all day at school. He gently told her that it's not true because he'd only just made it up. She kept saying "yes it did happen" in a quiet, broken, and shaky voice, which is how she sounds when lying. She just kept repeating it until he gave up.

When she tells the first lie in a story, she suddenly and momentarily freezes just as she takes breath for the next sentence. I don't know if it's that she takes herself by surprise or if she's trying to come up with how to continue the story.

Her dad sometimes tries saying "that didn't happen", but naturally, she gets defensive and doubles down until he gives up arguing. He barely knows her anymore and it's hard to get to know her when nothing she says is true. We want to enjoy talking to her, but it's mentally exhausting listening to what she makes up as she goes along. We'd just love to know how her day actually went, no matter how boring it was.

Has anyone else experienced this? Can she get out of it with therapy, or with age? She's going to end up being the one who's ostracised, and people will hate talking to her. She needs help, and I don't know how.