A close friend is waiting on results of an MRI scan which is going to take a long time, and although doctors believe she has spondylitis (not full blown ankylosing spondylitis, an offshoot of it) I think she is worried it may be bone cancer…
Can anyone tell me if these 2 illnesses are obviously different or if the doctors are perhaps predicting the lesser of 2 evils so as not to worry her in the meantime?

Ankylosing Spondylitis is where the cushions or discs in the intercostal spaces of the spine, lose their sponginess and become hard and rigid. it may further degenerate and the discs simply crumble away.
If that happens, an operation is performed to fuse the bones together permanently. it is a very painful illness, resulting in rigidness and stiffness when moving.
It is unlikely to be bone cancer, as cancer in the bones as a primary illness is very very rare. More often than not, bone cancers are the result of other cancers in other organs of the body spreading (Metastasising) to the bones. the most likely of these cancers is Lung, prostate and kidney.