I'd just sit tight if I were you and wait the 18 months. There's a far higher chance that your DSD will spontaneously desist than that she will still be keen to transition in a year and a half's time. Time is your friend in this situation.
As other posters have said, your DSD is far more likely to be lesbian than trans, and no one should encourage a young person to go down the route of transition. It turns a healthy young person into a lifetime patient. It's particularly destructive for girls because testosterone does more damage to women's bodies than female hormones do to male ones, and binders can cause very serious health problems, including lung damage and even gangrene. Only a couple of years on testosterone can cause permanent changes including male body hair, deep voice and male pattern baldness. Then of course there's the loneliness. Most of us spend a lot of time looking for love, for a partner. Being trans inevitably makes the process far harder, simply because your pool of potential partners is so much smaller.
It's hard to know what you might say to her but talking about what being a woman and what being a man mean to her might be a start. The recent survey of 200+ young women who have detransitioned found that their motives for wanting transition had never been explored by counsellors who, without exception, promoted the trans agenda without reflection.
Reasons for deciding you want to be a boy might include internalized misogyny and/or lesbophobia, eating disorders, a history of abuse, disgust at the porn-based objectification faced by young women today, and ASD. So many transmen have ASD it's almost a feature. There are lots of reasons that give what you might call a "false positive" effect for being trans.
Other posters have directed you to useful resources. Your DSD might be interested in the many Youtube videos by detransitioned women and also, depending how interested she is in theory, the survey. I can't link to it because I'm on my phone but it's easy to Google. Jen Bob's video on hormones for young people is very frank, and funny with it.
The other angle is her social media use. It's social contagion that's behind most teenage claims to be transgender. There are several to a year group in some schools and this is fuelled by social media. Groups tell each other they're trans, swap advice on what to say to counsellors, and never, never seem to agree that it's wise to give yourself time. Older people often seem to be encouraging this in online forums. Trans is where it's at for fragile teens. It appears to be the new trend that's replaced self harm. Self harm was dangerous enough. Makes you
look back nostalgically to the days when unhappy kids became Goths and Emos.
I wish you luck in a tricky situation.