Some UK seats have a tether strap. I observed a UK seat in action a while back and mom showed me how it was installed (ISOFIX and tether) and attempted to show me why her UK Britax was better than her Canadian Britax Frontier. Eta: just pulled documentation -- it was a Romer Duo.
Then I showed her how to actually install her Canadian Frontier. And got it rock-solid in under 30 seconds. And she was delighted and relieved. And I saw a gimmer of something in her eye -- like maybe non-UK seats aren't junk afterall.
I have no idea if she changed seats, but I do know that based on my perusing her manual and watching her install her seat, and seeing her tighten the harness to a beautiful and rare snugness that seems to be inherent on seats that have no chest clip (I don't think I saw a single harness left super loose on an infant seat when I was in Europe for two weeks), I was pretty sure her seat was installed and being used correctly *per the manual*.
Still very much illegal for use here, a ticketable offense, and strongly discouraged, but it was in fact pretty interesting to see a seat being used correctly, so easily, practically zero effort, and for that I was quietly humbled. I can only imagine how strange it would be to be used to say, the ease of ISOFIX, and then come here and buy a $300 Canadian seat of identical brand, and struggle like the dickens to install it AND have someone saying your identical brand easy as pie seat is illegal for use. It's a tough sell, for sure. Add in a message board (in your case) where people are telling her it's fine, and well, good luck.
I ramble, my apologies. I'm not sure what to suggest. People make choices and sometimes face the consequences. I'm not sure you (on a message board) will convince her the seat is unsafe for use, but you can stress the legality issues. After that I think you just to let it be.
Eta: on the off chance that she does re-consider buying CMVSS seats, you should maybe consider validating the fact that she might find them harder to use than her current seats. And it's a good idea to mention that we do have seats with rigid latch (on the off chance she prefers it), and I think it would be awesome if you could get her in touch with a good and understanding CRST who could possibly walk her through some seat options, maybe offer to meet with her and discuss things, show her some CMVSS seats, the similarities, and the differences. Our seats don't bite.
Thinking about moving to a new country where things are so different and I might cling to what I know too. But if someone actually reached out to help with the transition... well, that would be very helpful... less scary.
Again, rambling... apologies...