I agree with Karin, something has certainly happened in this person’s life to result in such a dramatic weight gain. The trick is how to approach the situation. Consider the following:
1) how close are you to this friend? is it common for you to discuss personal issues?
2) are you familiar with her personal affairs (issues/happenings that may have contributed to the weight gain)?
3) she may very well know how to make healthier life choices, but is now choosing not to, or is simply unaware of her choices.
4) do you have mutual friends/people you know that can act as support?
I think, above all, if you truly care for and respect this person your words and actions will be understood as coming from a good place; whether or not she is ready to commit to a new path will be the key. Perhaps, more than anything else, she’ll need your time, support, patience, and love.