Tread carefully, brother. It's gonna be hard to talk somebody out of a transition, and it might even end your friendship or result in him hardening his resolve to transition.

Also, I don't think transition is always a bad idea. Blaire White might be a self-hating gay man, but she seems happy living as a woman and she understands she is biologically male + she's not infringing on women's rights so I say all the more power to her.

One thing about puberty blockers. If your friend goes on puberty blockers followed by cross-sex hormones, [s]he WILL be infertile, permanently. Puberty blockers prevent a male puberty, but cross-sex hormones don't magically give you a womb and eggs. 15 is VERY young to decide to never have biological children ever. Here's a source from an extremely pro-trans/"affirming" hospital, UCSF: https://transcare.ucsf.edu/guidelines/youth

The consent process for hormones should include a conversation about fertility. While options are being explored to preserve future fertility for transgender youth, the current reality is that cryopreservation is very expensive, in many cases prohibitively so for those with ovaries. For youth whose pubertal process has been suspended in the earliest stages, followed by administration of gender-affirming hormones, development of mature sperm or eggs is unlikely at the present time

It's hard because your average 15 year old doesn't want kids and thus doesn't understand the gravity of sterilizing oneself. Maybe you can show him the Blaire White videos where she talks about how she's glad she transitioned as an adult because she can still go off hormones and produce sperm if/when she wants to have biological children.

Finally, if he does want to get the surgery, the blockers/hormones are a bad idea because they leave you with not enough penile tissue to do the penis inversion technique. Here clips of Jazz Jennings and Jackie Green might be instructive. Both of them had terrible SRS experiences due to transitioning too young. Jazz in particular is incapable of feeling sexual arousal and has never had an orgasm. It's tantamount to child abuse in my opinion.