Hi Blessing. I’m assuming in your question that you actually mean to “improve” her hip mobility – true? There are many ways to accomplish that, but always, take into account your client’s current limitations and the recommendations of your client’s medical team (e.g. physician and therapists). Easy cycling on a stationary bike, passive and active stretching, and progressive resistance exercises with bands (such as hip flexion and extension) are all exercises that I’ve used with my clients with post hip-replacement mobility and strengthening.
Take your time, listen to the feedback of your client, and communicate with your client’s healthcare providers and you and your client should be fine.
I hope that this helps.