This is a difficult area to address. There are several possible reasons reported for dislocation after total hip replacement. The incidence varies from 1- 10%. Studies have reported that 40 - 70% of the dislocations occur in the first month after surgery. The reasons for dislocation could be many: To list a few:
a higher risk of dislocation to the back of the hip is reported when the hip is approached through an incision of the back of hip,
orientation of the cup or the ball (head of the femur) could be a problem, some times both the cup and the ball orientation may contribute,
in elderly persons when the muscles are very lax, the hip could dislocate easily. Normal muscle tone is necessary to keep the head in the socket.
Some hips dislocate without any obvious reason.
It is said that if the hip dislocates within the first month, chances of re-dislocation later are less. However, if there is recurrent dislocation in the beginning itself, one will have to look for a specific cause. It is difficult to suggest the reason without seeing the patient or the X-ray. Treatment depends on the underlying cause(s). If it is as a result of the surgical approach chosen then reducing it and maintaining it in the reduced position through braces or other devices for 3 weeks or so may help prevent re-dislocation. If it is as a result of orientation of the cup or the ball then there may be a need to revise and re-orient the cup (usually) or the ball (rarely).
If no obvious reason is found and it seems to be dislocating, some surgeons tighten the muscles of the hip that help to take the hip out to the side (trochanteric advancement).
Weak bones often result in cutting out of the metal implant from the bone. If this happens then surgical intervention is required. I do hope your mother-in-laws problem is resolved without any surgical intervention.