Here's the thing, if you're friend is worth being a friend, she is also worth sharing your life with. If you loose her as a friend over this, was she really your friend to begin with? The honest truth is, EVERYONE has something in their life they think they have to hide, it just so happens, ours is a little odder than others, but only because it is more unusual, not because it is somehow worse. The greatest thing is, when someone is willing to be vulnerable about something, it instantly opens the door for the other person to be able to share their "stuff" too!
My advice, when you meet up with her again, just wear a diaper like you were last time. She may not say a word, so don't stress about it, but if she does, just be honest, start with the basics and if she seems receptive, open up to her... And if she isn't receptive leave it simple. If she somehow decides she can't be a friend any more, which is pretty unlikely, she honestly isn't worth being friends with anyway. In the end, most likely it will end up bringing the two of you closet together!
Best of luck to ya!!!