Usually a lot of humerus fractures can be managed without surgery but we will have to evaluate online if there is any neurological deficit associated with the fracture and since your mother is already on chemotherapy it might be advisable to follow conservative that is without surgery option kindly consult online by booking appointment

Humerus cast initially

Please share another x ray ,there seems to b cortical breach in elbow too. To rule out  fracture in elbow get fresh x ray  elbow AP and lateral view. Its seems to be a pathological fracture. Its pattern suggests it to be a pathological fracture  until proves otherwise.  She is already a patient of CA breast, high chance of secondries.  Please get full investigations like full body bone scans, chest CT, Abdominal ultrasound,  certain blood investigations. It's better to get operated and a intramedullary nail spanning whole humerus is to b done . Otherwise in non pathological fracture of this type r best treated conservatively in cast / plaster. I prefer extension cast with excellent results.