I can’t say that I have much expertise in Qi per se. My understanding is that it is a very vast subject, like talking to an M.D. about blood. In Taijiquan, I think that Qi is supposed to be mediated by the soft tissues in the body, i.e., tendons, ligaments, etc. Hip replacement surgery mainly removes bone and should have no effect on these tissues, assuming the surgery was uneventful.
As you may know, the bones in the body are indeed live tissue that does and should remodel over time, depending on diet, exercise, and general health. The metal, plastic, and bone cement (if any was used) cannot do this; however, the remaining bone should be largely unaffected. Good health requires a balance between keeping the musculature and bone strong and not causing premature wear to the plastic and bone cement that would loosen the placement of the prostheses.
You may also have heard about Qi “permeating the bones” or the “bone marrow.” Again, I do not think the surgery does much to the tissues that are said to be involved in this process, other than slightly reducing the available amount of bone and associated marrow.
I hope this helps.