In all the transatlantics that I fly with my kids, I only once did I not have a car seat (I didn't make the reservations that time).

They also accept U.S. seats because they make reference to it (although they don't state it outright lol!);

“This child restraint system conforms to all applicable Federal Motor Vehicle Safety Standards" and “THIS RESTRAINT IS CERTIFIED FOR USE IN MOTOR VEHICLES AND AIRCRAFT" in red lettering.

http://www.aircanada.com/en/travelin...ant-child.html

There are quite a few car seats sold in both N. America and the U.K. which obviously are approved for both. She can contact the manufacterer of the seat she already owns to find out if it is approved. I know that the Britax's are almost all FAA approved (except the Regent).

Of course, she can do what I often do and just board with the seat. I strap it in and see if there are any objections. When I was a Flight Attendant, I never checked seat approvals on principal. I felt strongly that this was the parents' responsiblity and I was so glad to see a child flying safely in a seat, instead of in a lap, that I was not going to give them any grief.

Just as a tip, for a toddler, I check the stroller (or arrange one at my destination) and use the car seat for that purpose. I strap the car seat on to a small, metal, foldable cart with an extra bungee cord and this is much less expensive than those Gogokidz carts they sell. It honestly works just as well. Use a backpack instead of a diaper bag and it can go in the seat when the toddler walks.

My car seat weighs 25lbs and it's no problem to bring it and I'm alone with three kids on transatlantics.

Good luck to her and good for her for buying the seat for her baby!
other non-commerical flying tips;
http://flyingwithchildren.blogspot.com