It's tough, I know. My friend is constantly making mistakes with her kids/seats. I quit saying anything. However, I won't ride with her either anymore. And if she rides with me, the kids ride in MY seats, and I make sure they are in properly.
The last time I saw the kids, she brought the baby in his bucket. It's a Snugride, with a 3pte, that's been in a wreck (although VERY minor), with a thick coat on, straps loose and a belly clip. He's 18lbs @ 6mo...so I keep reminding her it's almost time to move him to the convertible. She kept the last one in the bucket WAY over the height AND weight limit. I won't quit on that matter. The rest...she'll have to live with the consequenses. Would it break my heart? YOU BET. But she's the parent...and if she chooses to live in a bubble...all I can say is I pray it never pops.