Has she had girlfriends or boyfriends before? I think it's crazy that so many teenagers are transitioning, many of whom haven't explored their sexuality yet, in the fully functioning body they've got! I have read that many post op trans people are disappointed with the surgery results.
How does she feel about the fact she'll have to use male spaces? Public toilets, changing rooms etc. Does she understand that as men and women grow up and settle down, the dynamics change? How differently she'll be perceived as a man, than she is now as an androgynous young woman? Does she have any clue how men communicate when (they think) there aren't any women around?
Does a trans penis look and function like a normal penis? I assume ejaculation wouldn't be possible, and she/he wouldn't have a clit, so she/he could she orgasm? Do they add fake testicles? If not, it'll soon be apparent to any potential partners she wasn't born male.
I really think if this had been a thing when I was 14/15, I'd have very susceptible to it, as a "way out", a way to make things better. I became a goth, self harmed, took drugs, drank a lot, questioned my sexuality and experimented etc for a good two years, before realising that my sexuality had nothing to do with my angst, just me trying to make sense of it, thinking being lesbian might be why I always felt so different and out of place. Hormones are bastards sometimes, and I realise now I was actually seriously depressed. But I'm happy in myself/my body now. I just turned 25, and what you're all saying about the still growing until then/feelings and attitudes changing is totally true. I've never felt so at ease with myself or content, as I do now. And I've just been through a horrendous break up (7 year ltr/dc together), so that's saying something. If 15 year old me could meet me, she would think I'm a boring, sensible, average, single mum, with a mundane job, and be disgusted (she would envy my pets though), and have a big moody, self absorbed, self pitying existential crises over it and been a Kevin to my parents. I'm so glad I'm not a teenager anymore.
Are any of the dc mentioned here on websites, getting dodgy info and advice? There was a big thing when I was a teen, for looking at self harm and anorexia/bulimia sites. There were "Ana" and "Mia" rules to follow, advice, etc. Dangerous, crazy stuff like "nothing tastes as good as skinny feels".