She could just do without the seat on the plane, some people prefer that. Also I think I read on a post here about Candians flying through USA, it only has to say approved for in flight use, it doesn't have to be a USA seat, maybe the same would apply for someone not living in USA. I have also heard flights leaving EU don't care what type seat is used (if a seat is used), it is only leaving USA they check and that has been my experience too.
I just really have come to believe that cars here (in EU) are different than cars in USA. The seats here have been tested for use on cars here. USA seats have not been tested on cars here. Unless she is going to bring her USA car (which is not totally impossible), I think it is a better idea just to get a seat here so that the seat and the car have a higher likelihood of being compatible. KWIM?
Which thinking about all of this has my head spinning and thinking... since I have a German DL (which never expire), if I were to move back to USA, use my ECE approved car seat, then get pulled over and show my German DL and say I am just visitng, could I get in trouble? (yes I know lying is bad and that I could definitel get in trouble for) but what is to stop people from doing something like that?