Are you and she on FB? If so, maybe share a link on there about the dangers of FF too early (a big SUV will do absolutely nothing for a child who is prematurely FF) and tag her and a few mom friends and say you just heard this and thought it was useful information for all moms. Otherwise I'd send an email with the same thing (don't single her out, act like you feel it's useful for all parents.... because it really is). If she doesn't change her daughter back there's nothing more you can do, especially since Peru doesn't have laws, but at least that would inform her.
I'm very big on carseat safety, so I would most definitely say something. I don't know what your relationship with her is like, but if you don't feel comfortable saying something to her directly, I'd go this route. I hope she listens to you and turns that baby around. Scary.