HOMLAND Toilet Safety Rails for Seniors, Adjustable Height & Width 350lb Toilet Handles for Elderly and Disabled, Foldable Toilet Safety Frame, Fit Any Toilet

They make ones without the bowl you can place around the toilet.

I had Bilateral TKA a few years ago. She shouldn't have issues with the space as her knees, although sore, will bend. I highly recommend you but in some grab bars to make it easier to get on and off the toilet. Something like this will also help.