I saw a modified knee lift somewhere online. If it's for work, she could contact the Job Accomodation Network. They have been a huge help to me. The standard bar, as others have said, simply would not work without use of the upper right leg. I have even seen pressed foot lifts that are operated with a bite bar so it's just a question of finding a good OT/PT and a machinist, welder or such to make an adaptive bar/extension for her. The standard bar would probably be a good start, but I don't know for sure.
I seem to remember there are some machines with a setting where the pressed foot automatically raises when you stop sewing. I can't remember the brand/model. It was way out of my price range. Perhaps a member has one. I would think that would be her best option.