Your friend probably needs a lot of support right now, and to feel that she's not alone. It might also give you a chance to reconsider whether or not you're okay with being eternally closeted, and give you the opportunity to be yourself as well.
Granted, I still don't have a lot of experience in this area, buuuuut.... Having a supportive trans friend would have caused me to come out of the closet way sooner than I did, simply because I would have felt like I can fully be me around them; something which they probably need as well.