Have they also tested her pituitary hormones - and did the MRI look at that? Was her MRI done with contrast? In what way is she losing her vision - central first or peripheral first? If it is peripheral first, please get her hormones checked by a pituitary specialist. The pituitary is right by the optic nerves and sometimes, well, the tumors are hard to see or it could be empty sella.
IGF-1, FSH, LH, ACTH, TSH, PRL, and Vasopressen should be tested. Really full hormone panels should be done but that is a start.