To tell someone they aren’t buckling their child right or to ignore it?

Say something. You wouldn’t forgive yourself if something bad happens

I would say something but try to be really nice about it otherwise she'll just be defensive... if I were you I'd say "oh I thought kids weren't supposed to be in a booster seat until age 6" (or whatever age it is I really don't know)

There was a gruesome crash recently in my city. A mom and her two kids (3 years old and 18 months old). The car was bent in half, all 4 lanes of traffic had to be closed for a good 2 hours. it was a nightmare. Something where you look at it and can’t believe anyone made it out alive. The mom had numerous surgeries and was in the ICU. The babies were fine. A broken bone on one and a few stitches for the others. First responders said that had those babies been buckled in incorrectly, they would have died. The ONLY thing that kept them safe was properly secured rear facing car seats and a properly buckled harness. I’d say something. Because even if she thinks it’s “no big deal” and throws the famous line “don’t tell me how to raise my kids”... what she is doing IS putting her baby in danger. Car accidents are no joke. But that’s just me.

Definitely say something maybe she just doesn't know that what she's doing is so dangerous

Say something and maybe hurt an ego > don’t say anything and possibly risk the life of a childHow is this even a question???

I'm gonna have to agree with most everyone else, say something for sure, but try to do so without making her feel stupid or inadequate. Or, ask her to come with you to a child seat safety check event and then the both of you could go and maybe have lunch afterwards, and that way the child seat safety specialist can educate her and teach what she needs to know! Then, you won't have anything to really feel guilty about. 

Definitely say something

You can say something without sounding overly condescending. A simple "Are you sure that's right? I was shown a different way that's supposed to be safer" would be good! It's always awkward giving people personal advice but it has to be done

You don't have to directly say 'what the fuck are you doing you moron' but you could make up a general story about someone's kid getting killed because of what she's doing. Act like you don't know she's doing it.

I'd nonchalantly bring up the booster seat and say something like...'oh!, I thought they just came out with a new safety guide/law that said that was dangerous and they should be in (insert pertinent information to child seat safety)!' If you go into it getting aggressive by telling her she's doing it wrong she will most likely take it as a swipe at her parenting, shut down , and tune you out. If you've addressed the issue in a thoughtful and careful way and she still pushes back then there isn't much you can do after that outside of contacting authorities. Which is a pretty bold move.