Obviously she knew you wnated her in the carseat. You gave her your whole car for that purpose. Some people just don't get it.
As for the guy: I would cut him some slack. First of all I loosen the straps and slide down the chest plate when we are hanging out in places because I want dd to be comfortable and I fail to believe that any sort of restrictin can be comfortable. He may have tightened up when he got to the car (i doubt it but we can hope) Also someone here said it doesn't matter if thier legs are hanging over so long as thier head doesn't go above the top of the seat. The leg thing gas always concerned me but I guess it is OK.
My SIL came over one day with her niece. They got where we were going first and neice was sleeping in the guest room so i didn't see her until we were ready to leave. She was easily 14-18 months 25-30 pounds and still in her infant seat. Wearing a winter coat no less. And SIL brother hadn't bothered to give them a carseat lesson (BIL and SIL don't have children yet) since they didn't have children it was easy for me to give them a lesson without offending - they knew it didn't look right. I must have spent 20 minutes adjusting straps and rethreading to make it fit her but obviously she never rode in it right. I don't know what I said )(I know I mentioned she was far to large for that seat) but SIL called me a couple of hours later and asked whay kind of carseat they needed to buy because they were buying one that afternoon. I don't know if SIL or her brother was buying it but they took my word as gold. I think they might have even been calling me from the store.
My point is that sometimes people don't know what they are doing is bad (I know, they could read the freaking owners manuel) but if they are a reletive, babysitter or a non custodial parent they might just not have the information they need to do it right and I don't think they will be offended if you gently help them. heck they may even be thrilled that thier child is now safe.
Oh and one of the worst things I have seen is the infant seat turned around to be forward facing. Wrong and dangerous on so many levels.