Sorry OP I made an assumption and abbreviated - the Cass review is the review of The Gender Identity Development Service (GIDS, or the Tavistock and Portman clinic.) It's being done by Dr Hilary Cass.
There is so much for you to read and I don't want to overwhelm you with multiple links. Waiting lists for treatment due to the high number of referrals will cause a delay, but if you seek advice from your GP they could refer you to join that waiting list. Your DD is an adult so would not be required to seek parental consent but it's great she wants you with her and 'watchful waiting'
is far better than rushing into something she may later regret.
Puberty blockers would have little effect due to her age so I'm assuming she would be seeking cross sex hormones and want to discuss possible surgery (double mastectomy) with the Dr but is she keen to have therapy? I really hope she can find someone to talk to rather than rush into the medical route.
I would also suggest Stella O Malley maybe contacting her - she did a programme for Channel 4 called Trans Kids: We Need to Talk.
cass.independent-review.uk
Though your DD is older than I thought the referrals for girls are far, far higher than for boys. Has she said where this comes from, has she shown any interest in this before, at a younger age? If not then it could be Rapid Onset Gender Dysphoria and for this I would look at the work from De Lisa Littman
journals.plos.org/plosone/article?id=10.1371/journal.pone.0202330
Her work was suppressed and she received huge criticism for even exploring the topic but she persisted and I think her work is hugely important in this conversation.
Without scaring you too much I would recommend that you look as the Detransition Advocacy Network, Pique Resistence and the newsnight coverage of GIDS and Keira Bell and see if you can get hold of a copy of Irreversible Damage by Abigail Shrier.