This morning, because of the weird MRI results, Sara got a brain biopsy. They removed a needle-sized chunk from her head and looked at it. Histology is negative for lymphoma; prospects of high-grade astrocytoma are (apparently) reduced based on this morning's MRI. This does not eliminate all of the really, really bad diagnoses.
This does, however, reduce the prospect of things which kill my partner and your friend. Our hopes are better now than they have been.