From talking to the family air ambulance travel was denied and the ambulift was used when disembarking on the return leg when the weather was similar. She has been travelling by air ambulance since the incident. The pax was able to move from lift to seat but she was not told about the step in the middle of the plane and tripped.
I don't really know what you mean by not their responsibility. It clearly states on the website of most airlines that assisted boarding is available. If you ask for it and it is granted surely then they are responsible?