Hmm.....its a tough one but YANBU to be concerned for this little mite. If you feel the hubby is a bit unapproachable (reading between the lines of your post), then why not try chatting with the mum and asking if there was a prob with the seat after you fitted it? Just a way of introducing the topic without being accusatory.
You would be surprised at the number of people who do this. I cancelled an outing with my (very posh and intelligent) aunt because she rang at the last minute to tell me there would not be enough room in the car for dd's car seat and, (I quote) said "I thought I better tell you now that if you do come she will have to sit on your lap for the 90 min (motorway) journey and I know that you're really funny about things like that, alhough your cousin (princesstippytoes) doesn't have a problem with it. She almost made me feel guilty for being awkward!
Hmmm, could you quote the law to her and let her know that its illegal - maybe take it from the point of view that you wouldn't want her to get into trouble rather than questioning their verydubious decision?
In your shoes I think I'd try to steer the conversation subtly around to the car seat issue the next time you happen to bump into her.
Use MrsJohnSimm's tack, and throw in some made-up story about friends of yours being fined/whatever for not having their child strapped in when stopped at some lights next to a police car (something I would quite like to happen to some friends of mine who never strap their children in on the school run, despite the kids' Daddy being a paediatric surgeon)
lol, i remember a recent conversation with my mother along the lines of me screaming, 'its the law! mother. do you want me to break the law?' and moms solution being that the kids could go int he car seat, and i could sit on her lap needless to say, i stayed behind.
er, not a made up story at all. i was fined for not having five year old ds in a car seat, and on an adults lap, before the new laws became laws a coupl of years ago. stopped at red light next to police car, and that was me fined...
It's an infant carrier, one of those ones that you just put the seatbelt round and then take out easily.
But the belt only just fitted after much tugging and rearranging. I suggested they leave the seat in there and just thread the baby in (that's what I did a lot, but then I'm lazy) but they obviously don't think that's an option.