As Prudie pointed out, your friend might have ditched the bra because she was experiencing physical discomfort - and as you pointed out, only her friends will tell her that she really needs one.

Don't go the route of sending her a card, though, because I guarantee that will hurt more then help. You, or one of your circle of friends, needs to talk to her in private. Not all of you at once, because then she'll feel like you're ganging up on her. It should be a one-on-one conversation.

Tell her that her decision to give up wearing a bra is a little worrying, and that you're not sure it's the best thing for her. Remind her of the incident with the pictures that you mentioned in your letter. Then say that, while you don't want her to truss herself up on your account, there has got to be a better solution than simply going bra-less.

Ask her if she started experiencing physical pain or discomfort from her bra. She might have some deeper physical issue that needs to be addressed by a doctor. Or she may just need a custom-fitted bra. They're expensive, but a friend of mine who's always had difficulty with her own brassieres said that they're well worth it if the standard equipment doesn't work for you.

Let your friend know that you're here to help her work out whatever is going on and help her find solutions other than being uncomfortable or hanging out all over the place.