My heart breaks for your DD. I understand to some extent as well.
My stepdaughter also 15 (who I raised pretty much single-handedly since she was 4) is so upset with WH right now. She's hurting for herself, her siblings, and me. She's angry that the unfairness of it all.
Her parents marriage ended because of her mother's infidelity and now she's faced with her father's as well. I wish I could take away her pain, but it's impossible to do.
WH feels that it's not fair that she's sided with me when she's not my biological daughter. He had convinced himself that he could just sweep another woman through her life and she'd forget all about the woman/mother who raised her.
So very sad.