I think one of the ways to talk to her about breastfeeding, is maybe to point out that breastfeeding consumes more calories and thus make it easier to loose the baby weight.

Otherwise, all you can do is be there to offer advice when its asked for or needed. It is hard to bite your tongue when you think the person is doing something wrong; but unless it is dangerous, I think you do need to only talk when they are being receptive or when asked. Having said I would personally have to say something about CIO / CC, I do believe that those methods can be used successfully, but NOT before 6mths old!!! Certainly not with a newborn! That is neglect with a newborn!
But at the end of the day, you cann't be responsible for others decisions.