I totally agree with MM, be sure to be there for her, though you might not understand everything. She will need you and all the support you can give her to go through all these changes in her life. It's a big turn in her life, and she will need to see that she can rely on you (and her other friends and family), because after all, it's her LIFE that changes, and the way she's living it, NOT HER. She's the same exact person ; she will just be a married girl to the man she loves. Even though that implied to change things.
Wearing a sari won't make her a bad friend, or someone you can't rely on and talk to.

And don't forget that as long as she's happy, the rest (almost) doesn't matter. :)

Something similar happened to me (although it was not religious) a friend drastically changed overnight and I didn't know how to handle it. Suddenly you feel like you don't know her and everything you used to do and talk about now feels completely out of place and irrelevant. You feel like you've lost her. It is like waking up one day and theres your best friend but its not really her and you just want to give her a big shake and say 'I KNOW YOU'RE IN THERE!!!!'. I think the best thing to do is speak to her about it, tell her how you feel (in a good way), tell her your insecurities but remember this is the life she has chosen and you should understand that and support her like you've always done. She's still your bestie and she'll want to know what you're thinking because she cares about you. Just let her know that you're finding it difficult to know how to react, or know what to say to her - she'll get it. Being open about everything makes it alot easier and it cuts alot of that awkwardness out. Good luck.

Maybe the person she was before is not who she wanted to be, and now she's finally happy with herself.

I flipped from religion to religion until I finally admitted to myself that I'm an atheist. This has upset my mother, alienated my grandparents, and made me a rather more depressing person to have religious/spiritual conversations with, but two decades of pretending I believe in god has been exhausting.

To a lot of people, they probably think this isn't me, because we tend to think that the image we're presented with the first time is the "real" them, and we don't like thinking that people don't trust us with their real selves. But most people feel the need to hide themselves for the majority of the time. Maybe this friend is being her real self now, or maybe it's a stage of life she needs to go through to make sure of who she really is. Either way, support is best. :)