Unfortunately I have ongoing foot issues, so I can certainly relate!
I have a heel spur that comes and goes. When it flares up, anything that puts pressure on that heel is uncomfortable, so I only use my spin bike or rowing machine for cardio. (I would definitely recommend rowing as an option but would warn that it can get boring after 10-15 minutes.) I prefer the spin bike to a regular stationary bike because I can stand & "jog" on the pedals without having any weight on my heel. (If her spur is not on the heel, this might not be helpful but I thought it was worth mentioning!)
I frequently alternate cycles of rowing with spinning. I often sit up straight on my spin bike, and use it as more of an upright stationary bike without leaning forward in the traditional spinning position.
When the spur starts to get better & I am feeling my plantar fasciitis more than the spur, I like to use my bellicon. Doing it barefoot helps stretch out the muscles/tendons/fibers in the bottom of my feet. But for a full-blown spur it can be uncomfortable.
Sorry to be so lengthy. I hope this helps a little!