Two thoughts- rowing on a high quality rowing machine can be pretty easy on the knees, but only a physical therapist or MD Orthopedist can tell you for sure.
Point two, in my experience many highly qualified dieticians have a tendency to espouse only the eating plans and "change" strategies in which they themselves have been trained.
For SOME of us, it is necessary to research different patterns (vegetarian, low cab, hi fber, Paleo, etc, just to name a few, and MANY others) before one approach emerges that works for your individual needs.
It is part of your search to find something healthy and nutritious that WORKS FOR YOU. Be prepared to be TOTALLY HONEST (do you binge? Have food cavings?) sometimes behavior that is actually "eating disordered" contributes to long standing obesity when we're not even aware of it!
Wshing your sister LOTS of sccess!