Wow! What a SIL to have. And when you're tired, postnatal and vulnerable, it's just so hard to stand up to anybody, let alone a bully.
Not sure when your friend is due, but if she's got time, I'd recommend she see a counsellor just to work on assertiveness skills and how to handle the SIL and other family members. It's easy for outsiders to say 'why doesn't she just tell her SIL where to go?', but it's so hard in the moment to stand up to family when they're so opinionated etc.

She shouldn't be made to feel like her C.S is shameful or need to be kept a secret. I hope her partner is supportive and will stand up to family for her, when needed?
And breastfeeding after a C/S is possible -it just might take an extra day or so for the milk to come in (different for everybody, I suppose, but that was my experience).
Good luck to your friend and hopefully, she will have a much, much better birth experience this time around