Has your mum had anysort of trauma to the hip...Has she fallen? Ever broken it? Has avascular necrosis (avn)ever been mentioned? I had/have avn, but it comes on througha trauma injury such as a a break.... It's basically a bone disease, ....When a bone breaks, it repairs it self until the break has rejoined...With avn, the bone continues to die and crumble faster than it can reapir itself....If that makes sence..This could be why part of your mum's femur is missing.
Not being a doctor or medical professional, I can't even guess how this has happend, however..It has happend to me, and becuase of it, I recently went through a 3hr op to replace it.
Hope the doctors find out what is wrong and can treat it!