I hate to dissuade you as I think it is a really nice thing to do to help a friend look and feel her best. I know however how hard it is even for myself to find one, even when I try them on, I can't imagine someone else being able to find one in the right size EVEN with the best of intentions. As you describe her in the picture it is an indication that if you asked her her size, it sounds like she either does not know what size she is supposed to be wearing, or the bra has worn out (she has not noticed or does not care), so you might be up a creek even if you asked her size. You could however broach the subject with her about how you went or need to go shopping b/c you noticed how some of your own bras are wearing out, they don't fit like they should, it would be nice to get a good fitting again, etc. and see if this opens the conversation up on her end, may she might get to thinking about it. This might help you to help her inadvertently.

As a friend, I would appreciate your concern and in wanting to help me feel and look better, but this is something that I think we have to do for ourselves just because of how they fit. It is really nice of you though.