www.transgendertrend.com is a good source of info on how to talk to your daughter about all this & to the other child's parents, maybe start with the resources for parents & for schools. There's also a book aimed at parents by Sasha Ayad, Lisa Marchiano & Stella O'Malley, When Kids Say They're Trans that you might find helpful.
Hilary Cass' report is worth anyone's time, webarchive.nationalarchives.gov.uk/ukgwa/20250310143642/cass.independent-review.uk/ particularly in this case with regard to social transition. It is not a neutral step, it is a powerful intervention & should not be embarked upon at the behest of amateurs or ideologues. Despite the flabby govt guidance, the reality is that school teachers are not psychiatrists or clinical psychologists, school counsellors would be operating far beyond their competence in advising (or supporting on a child's say-so) any social transitio.n Schools are not insured to diagnose, or to accept & act upon children's self-diagnoses, or to recommend clinical pathways such as social transition.
Ben Ryan's Substack benryan.substack.com/ gathers a lot of relevant US stories together & the parents might find the articles on US detransitioner lawsuits particularly interesting, they are, er, surprisingly under-reported in the UK the court documents are worth a look. Note in particular that the long-vaunted "medical consensus" in favour of underage transition isn't there anymore.
Although commissioned by noted whackdoodle RFK Jnr, America's HHS report on gender dysphoria was conducted by sane & competent people & is worth reading; opa.hhs.gov/gender-dysphoria-report.
As an ASD adult whose own gender woes did not resolve until my early 20s, I second the suggestion of Hannah Barnes' book. Dr Az Hakeem worked with adult trans patients at the Tavistock for many years & his book, Detrans, though more confronting, has some useful things to say about ASD & gender identities (It's not widely sold so try Amazon). It's bad enough that we get misdiagnosed with Personality Disorders, getting misdiagnosed as the opposite sex is inexcusable.
If the other child's parents have even a scintilla of doubt (the "affirm or your child will die" threats do still happen so perhaps they feel they have no choice) https://www.bayswatersupport.org.uk/ might prove useful to them.
Only total ignorance of the Furry scene could account for permitting a child to be involved. It's a highly sexualised subculture with many crossovers into extreme fetishes & paraphilias, & is entirely unsuitable for minors.
I'm sure others will suggest all the useful stuff I've missed out or missed seeing myself.
(Sorry if any of those links don't show live for you, they all work if cut & pasted.)
Good luck!