I'm afraid that is the test BA apply and I think we have to accept that. As I've mentioned on here before, when I had mobility assistance (i.e. buggy or wheelchair) due to a stress fracture in my foot, I was not allowed on the upper deck and I was far more mobile than the OP's mother. In an emergency I could safely have moved as fast as anyone - it just wasn't good for my recovery to walk long distances, by which I mean anything that would take a normal person five minutes or more.