Understanding transgendered people?

[Gályndrá]

Just...wait for your friend to come out to you. If your friend wants to be referred to as he, then respect that. Use your friend's preferred name, and be supportive.
As for if your friend is indeed FtM, then if he had girlfriend he would be straight.
Transgender was recently taken OFF the list of mental disorders, and the "lifestyle" would be no different than that of a cisgendered man.
As for talk to your friend about periods, bras and the like, do what's comfortable. I have no problem doing that with my guy friends.
Most of all, treat your friend like a human being, and if your friend does come out as FtM, then respect him as a man. That's all trans people want from others really, is to be accepted and respected as their identified gender.