Make sure that she follows the transition protocol and doesn't try to take shortcuts. This would include finding a therapist, living full time as a woman for at least one year, and THEN deciding whether to have the various surgical procedures.

It's possible that your child has been struggling with this issue for 15 years or more, and has tried to "fit in" as much as possible, and to accept their situation, and at the same time, has struggled with the terrifying thought of having to live the rest of their lives in the male body. The good news is that he hasn't killed himself, and although he turned to drugs, he isn't to the point where he is trying to kill himself with overdoses.

Encourage him to get the therapist, and start dressing full time. If she wants you to help her shop for clothes, enjoy the experience. Let her make the mistakes and discover what works for her as a girl. She will want to dress sexy and too cute, but that will lead to closer scrutiny. Eventually, she will want to dress a bit more conservatively so that she doesn't always have to worry about getting clocked.

If she is transgendered, she will find herself feeling more freedom, more alive, more satisfied with her life, and will want to proceed with the other things she need to do to transition. If she is not, she will eventually get tired of all the hassles of having to do the make-up, diet, exercise, hair management, and so on, and will opt to go back to living as a boy/man most of the time.

She can't really make an informed decision until she has had the opportunity to live as a woman for at least a year. You might even want to take all his boy clothes and give them away, putting anything sentimental into storage. In a sense, it "burns the bridge", and encourages them to move forward boldly and powerfully, rather than the shuffle back and forth.

Encourage her to read magazines to learn make-up, hair, and fashion techniques.

If she is transsexual, and she sees how much you love her as your daughter, she will love you more than ever.

If he is just a cross-dresser, he will love you for giving him the freedom to discover that for himself.