This is unfair to dad. Because she is now so de-conditioned she has fallen several times and recently had a procedure to inject cement between her L-11 & L-12 disk, and is confined to limited activity and walking with a walker. My concern is for my dad who is a very active outdoor guy. Two of his loves in life are to work in his garden and yard, and to go fishing. This has been ongoing for a couple of yrs already and I have seen my dad deteriorate. Any suggestions?
Anyway, it's already been suggested that you help out with Mom so that Dad can do what he wants, and it's been suggested that an aide or companion be hired. So really, I'm just going to ditto the above suggestions and say that I agree those are really good, reasonable ideas.
In terms of spurring your mother to greater independence/activity, I think that ship has probably sailed. Getting older people to work at improving their functioning, if they're not motivated from within, seems to generally be a lost cause. It sounds like her doctor is also advising her to keep her activity limited at this point, rather than suggesting rehab or physical therapy for her mobility issues.
One suggestion for your Dad (if he's open to suggestions) is to get a home helper to wait on your mother so he isn't tied to the house. Or call on other friends/relatives who might be willing to hang out with your mother for a few hours so your Dad can get out.