It may be best if she had a medical exam along with an evaluation by an orthopedic surgeon. The doctors can assess her level of risk based on several factors. They will look at her age and general health (both mental and physical).
Any other conditions present such as high blood pressure, diabetes, and obesity will be reviewed carefully. Other problems that can interfere with anesthesia such as sleep apnea or asthma do put the patient at increased risk of complications during surgery.
Studies show there is a trend toward a higher rate of wound infection among obese patients after THR. The rate of dislocation and blood clots does not appear to be any higher than in the average adult group.
Long-term results to show the effects of obesity on wear and tear of the implant are underway. Results for large numbers of patients aren't available yet. When dislocation or loosening of the implant does occur, revision surgery is often needed. Results are less than ideal in obese individuals under these circumstances.
Your aunt may need a longer hospital stay and more rehab services. It may be necessary for her to go to a transition or long-term care facility before going home. All of these steps can add to the total cost of the procedure. This is an important, but often neglected, factor to consider.
Many obese adults who elect to have a THR despite these concerns report a very satisfactory outcome. The pain relief they get is often enough to help them increase their activity level. They can have improved function and remain independent longer.